Combine approaches and extras to create different narrative justifications for powered and unpowered actions.
Because of the limitations of approaches this probably won't be really viable for a long-form campaign story lasting months-worth of sessions. Still, if you're going to be playing shorter games (a month per campaign, tops) this is an elegant option. If you want to use skills, read this anyway because I'm going to bring skills into it at the end.
I've experimented with this for a werewolf game, actually: by using approaches, your characters can have the same problem-solving attitudes regardless of their form: a Forceful cowboy turns into a Forceful werewolf, and a Clever schoolgirl becomes a Clever magical warrior. When they change form, the narrative shifts to give them justification for using their approaches in more magical and combat-oriented ways: a cowboy can't bite people but a werewolf can, and a schoolgirl can't create magical illusions but a magical warrior can.
(This doesn't work with skills because skills represent what-things-you-can-do-ness while approaches represent how-you-do-things-ness.)
You can represent this shift narrative by clever use of aspects and extras. Aspects should generally be phrased so they're relevant in both forms, which can be difficult but gets easier with practice. It's nigh impossible to give generic advice for specific aspect creation needs like this; the best I can say is that focusing on personality and relationships makes it easier to keep aspects relevant between forms.
Now, extras! There are many ways to craft extras around this. (Extras are for when you want to give effects that stunts can't handle, either because it's too many stunt's-worth of effects, or because a single effect is too complicated or powerful for a stunt to handle gracefully.)
Extra: Moon Infusion
Permission: An aspect indicating your magical nature.
Cost: At the start of each session I'm in, the GM's pool of NPC Fate points increases by 1 for each action I can use magically.
Benefit: Because I am secretly a champion of the Moon goddess, once per session I can reveal my warrior form. When I do, I gain the aspect Infused with the Light of the Moon (with one free invoke) for the rest of the scene.
This gives me magical context for using my actions (like flying, and shooting rays of cleansing light). When you take this extra, pick the actions (Overcome, Create Advantage, Attack, Defend) I can use magically.
I chose to make the extra's cost a "make the NPCs stronger" effect (increasing NPC Fate points) rather than a "make the PC weaker" effect (reducing PC Refresh) because, frankly, it's more interesting to face stronger opponents than to have your own power balanced out. In play it's effectively similar: the opposition scales according to the power the PCs bring to the scenario. The exact cost may need a little tweaking depending on your game. I've borrowed the basic concept from the atomic-robo RPG, which is a wealth of resources for this sort of thing.
Don't worry about including "running out of power" type mechanics in these extras: that's what consequences are for. Just as a gunfighter might take a mild consequence of All out of ammo, a magical girl might take Overcome by doubt or Cut off from my power.
But what about skills?
I started with approaches instead of skills because they're easier and more obvious to use with transformations.
On the face of things it's still pretty straightforward: Just as having a loaded gun lets you use the Shoot skill, having sparklemagic attacks lets you use it too. However, a schoolgirl probably doesn't have a lot of ranks in Shoot, so we need a new level of complexity in representing the transformation if your game uses skills instead of approaches.
Extra: Moon Infusion
Permission: An aspect indicating your magical nature.
Cost: At the start of each session I'm in, the GM's pool of NPC Fate points increases by 2.
Benefit: Because I am secretly a champion of the Moon goddess, once per session I can reveal my warrior form. When I do, I gain the aspect Infused with the Light of the Moon (with one free invoke) for the rest of the scene. This gives me magical context for using my actions (like flying, and shooting rays of cleansing light).
My abilities are different when I'm a warrior: when you first choose this extra, shuffle my skill ranks into a different configuration representing the talents of my magical form (the new configuration must still follow all the game's rules about skill ranks and caps). Whenever I reveal my warrior form, my skills change to their new configuration. They return to normal when I do.
Now we've got a girl whose abilities radically change but her aspects stay the same--so she's still the same person, but she has a different set of competencies when she's transformed.
Note: All of my terms and page references are from my English PDF of CoC 7th Edition. In addition, I have directly dealt with the player behavior described, and I have run CoC, but I have never dealt with this specific problem while running CoC. First I will address the rules as written. Then I have some more general advice based around things I've tried which worked in other games.
The Rules As Written
There is no rule in the 7th edition which expressly forbids players taking turns on certain tasks in order to maximize the chances of success. There ARE rules which can help you remain in the spirit of the setting.
The section When to Roll Dice beginning on page 82, and the section Rolling Dice beginning on page 194 both provide some guidance here. There's too much for a full quote, but the the core of it is "There's no need to roll dice for everything," and "The Keeper decides when to roll dice."
Applying this here, first, you as Keeper will decide when a roll is appropriate. The players can ask, and if allowed and failed, they can attempt to justify Pushing the Roll. But you decide if one is allowed in the first place. More importantly it means you can decide when a roll isn't needed at all.
Player: "We search the shelves for clues."
Keeper: "A quick search reveals one book hastily replaced out of order, its cloth bookmark marking a specific page."
No roll. Just a successful search.
The Rolling Dice section also suggests rolling in full view, even for the Keeper's rolls. I think this creates a different dynamic than rolling in secret.
The rules for Spot Hidden on page 76 address this directly, suggesting automatic success for players performing "a thorough search" might be appropriate.
The rules for "More than one player rolling dice for a skill roll" on page 86 should help you decide when you allow multiple skill rolls from the group. One of those examples expressly states all characters in a position to potentially spot something get a roll. But those rules also give you some idea when separate rolls should be allowed, and even required in some circumstances.
Lastly, if you or your players have access to the Investigator Handbook, the Rules Advice beginning on page 216 has some useful information on how to approach the CoC mindset, including this tidbit on Accepting Failure:
Don’t be disappointed when you don’t win every roll. Accept failure—it
can take the story to unexpected places. Sometimes, in hindsight, you
might be very grateful your investigator didn’t manage to open that
cellar door.
General Advice
The rules for Pushing the Roll (I think this is what you referred to as "forced rerolls;") begin on page 84. These rules DON'T refer to multiple players, but I reference it here because they lay the groundwork for something you can try, which I have done in other games. Consequences.
When Pushing, the player must explain what gives him the impetus to make a new attempt, and then the Keeper must explain the consequences of failure. As Keeper, you could use this even when multiple players are trying. There are possible consequences to multiple attempts which could occur regardless of who's making the attempt.
- "A second complete search of the room will take time you can't afford. If you fail the Cultists will be one step closer to completing their ritual."
- "A second attempt to break down the door will definitely attract unwanted attention if you fail."
- "A second attempt to repair that engine might irreparably damage a vital part, making it useless until you can have it in a shop during downtime."
- "A second attempt to intimidate the guard may make him mad enough to sound the alarm and attack you."
Several of the above examples touch on the second point. Your players should always consider time. Time is a HUGE factor which will work for or against the players. Wasting time on multiple attempts at the same action is certainly one way to ensure time works against them. The enemy may be closing in... or getting away...
In play, I've simply made players aware of the time required to perform repeat attempts. Sometimes I mention a specific possible consequence, but others I just say something like, "Sure, you CAN all search in turns. Are you sure you want to take an entire hour?" Then, after warning them, let them do it if they want. And make it matter. "You find that piece of information that eluded you, but now you hear a key in the lock of the front door!"
Properly used, time can help you.
There's an old text-based computer adventure where all the events are timed. It's literally possible to "wait" through the entire game and the adventure will just pass you by. I'm not suggesting anything this extreme, but if you demonstrate the world isn't static while they're busy, it will encourage players to move along.
Finally, consider the necessity of the roll. The absolute best way to discourage re-rolls is to not require rolls in the first place, except when needed. We touched on this above as it related specifically to the rules, but it's something to consider all the time. Is a roll really necessary?
In play, I generally call for a roll when the result will be interesting no matter how it comes out. Failure can move a story along just as well as success. And failure can be just as boring as success in some cases.
As a corollary to this, I use a rule I borrowed from another game called "Say Yes or Roll Dice." As GM in most games, it's my job to let players have their way, OR to make a conflict out of it. I love to RP and interact, but at some point the players will ask, "look, is this guard going to get out of our way or not?" And then I will say "yes," or the scene will become a conflict and the dice will decide.
As a final note which isn't really GMing advice, just make sure your players are up for the game you're running. Your descriptions make it sound like you have a conflict of tone vs expectations. There is a... pressing darkness... an urgency implied in many situations of CoC. The looming horror isn't going to wait around for your team to make 6 separate attempts to do ANYTHING. So, what are your players expecting out of this experience?
Best Answer
So, here's my approach:
A custom skill can be bought at the cost of any 2 other skills. This includes skills a character gets as part of his class.
The custom skill must be some sort of role (sailor, soldier, blacksmith, whatever) but it must also have a setting specific context (i.e. a sailor must have sailed with a particular navy or merchant fleet or something similar). This doesn't need to be written down in the skill name (though it can be, if it's brief), it's just something the GM and player need to be aware of, both to tie the custom skill to the setting and to help answer questions of context when they come up.
This custom skill can now be used as any skill within its specific context. That is to say, that when performing shipboard tasks, the 'Sailor' custom skill can be rolled in place of Endurance, Athletics or anything else. When it comes to knowing strange lore of the sea it may be used in place of Arcana. In short, it is a superskill within that specific context1.
Now, this is in part why it's important to keep the context very clear. Without boundaries, it is entirely possible to make custom skills overwhelming, especially if you treat it as just geography. It is not that a sailor can use his Sailor skill for EVERY activity on a ship, just the ones that sailor's do. Thus, he might be able to use sailor as a perception base to spot what's wrong with a ship, but not to spot ninjas sneaking aboard a ship.
Still, this is easily addressed with clear communication, and because this system mostly works within the context of existing skills, it's minimally disruptive while still expanding the scope of what can be done beyond the existing list.
That said, here are some optional rules:
If a custom skill seems too broad but not broken, it might be purchasable for 3 skill slots.
A less potent version of this approach swaps in custom skills for a single skill. In this system, you roll the new skill when no other skill is appropriate, but if you roll a real skill instead, you gain a +2 to the roll2. If you use this rule, you can replace the racial skill bonuses with a racial lore skill which implicitly covers those bonuses and which also allows for knowledge of what Tieflings enjoy for breakfast (and, implicitly, make those skills available to people outside the race)
The least potent version is simply "Works like a skill when no actual skill exists". In this case, I would not charge for it, but instead give each character one for free as part of the background system.
Hope that helps.
1. Structurally, the skill provides the capability to perform actions, knowledge about the topic, and a certain amount of awareness about relevant data.
2 - I forget the bonus type, but it doesn't self-stack.