It sounds like you have a very acceptable fire-focused intention. I suspect one of the things that is complicating matters is the fact that:
The Mage Wizard can select 2 Encounter powers per level, so that offers some variety. It helps that the DM lets us waive the restrictions on the number of powers we can use per day. I think that's why he makes the monsters so strong in each encounter, so they don't get wiped out easily.
Therefore, much of the normal optimisation advise, which assumes that you're holding to the normal rules starts melting away as the vicious circle of buff and counterbuff begins. (I faced this problem in a "by the rules" game when the DM reacted to the party's increasing optimisation by ramping up monsters, which caused us to optimise more, which...)
From a pragmatic perspective, save ends effects suck. While much of the game is well modelled, there is precious little balance to save ends effects, and the pendulum swings back and forth: standard monsters have little to no defense, but elites and solos become effectively immune as the game design progressed through the monster manuals. It takes a very deft touch in monster creation (if you're creating monsters "from scratch" to respect player agency in the inflicting of status while simply not going "nope!" to either them automatically winning or to them automatically being ignored.)
I, personally, have always enjoyed the more controlly-type controllers, and so my wizards, druids, psions, and invokers have focused on debuffing and forced movement. So long as you rely on effects that are more difficult to shed (either being end of next turn or encounter long) then you can focus on being to reliably land them, rather than inflicting sufficient debuffs to the monster's saving throws (that'll only be countered by the next monster) to maintain the debuff. The same thing is true in the other direction. I've played paladins who granted +9 to saving throws by smiling. This led to the DM completely foregoing the use of save-ends effects until the DM and I agreed to voluntarily limit that feat to a +5 bonus.
My recommendations are:
Nothing is as powerful alone compared to a party that is designed to work together.
Stop focusing on solo optmisation. It's a trap. Instead, try to make sure the party is designed to work together to achieve your desired requirements. Everyone will have more fun, and you're unlikely to bear the brunt of your DM's nerfing alone.
Have a side conversation with your DM: Explore what debuffs he's comfortable with.
Boundary setting is important. If you have a chat over coffee as to what he considers reasonable, you won't find the powers nerfed in the middle of a game. Set up, describe, and agree upon expectations for your character's capabilities such that he knows what to expect (such as to provide you maximum Fun) with the minimum of unpleasant surprises. As 4e is very much combat-as-sport, the joy is in the execution of plans within a chosen narrative (yes, story matters, to provide a need and justification for mechanics) than it is finding unusual solutions to the DM's prepared set-piece battles (many other systems are far far better at simulation).
It's very hard to alter characters in midstream without a retcon. Be honest and do a proper retcon, don't just knudge.
A character is the combination of her parts and their interactions, not just the parts alone. If you're changing a character's rasion d'etre, be honest about it, and change the character completely to fit your new requirements.
Yes, of course! Whenever the fictions establishes it!
That means, whenever a PC (or NPC for that matter) is convincingly able to hit multiple targets, it can do so.
How do you handle that mechanically? You have the PC roll any defy danger and/or hack&slash as usual, let them roll their damage and apply the damage to every target hit.
On/around page 56 in the rule book (explanations below the Hack&Slash move) it is stated: "If the action that triggers the move could reasonably hurt multiple targets roll once and apply damage to each target (they each get their armor)."
Now what are the implications and consequences? Dungeon World is all about going with the fiction. High risk or difficulty of a task is not reflected by the roll (hence you will never find a difficulty adjusted roll in Dungeon World), but by the consequences on a miss. A miss on such an attack provides the GM with a truly golden opportunity for a hard move. As the PC put a lot of risk on the table, you can then choose to answer with an appropriately scaled harder hard move.
An example from an actual game of mine:
The group had just opened a big stone gate by solving a dwarven riddle. Behind it, some goblins were also trying to open the door (to escape the cave that was the lair of an undead chimera). When the groups' s Barbarian saw the cluster of goblins, he said: "I take my 2-hand sword and spin around, whirling through the crowd!" I had him roll defy danger on STR, because he was powering through a horde of goblins. He rolled a 7-9 and as a result had the choice to lose control over his Barbarian rage or get overwhelmed by the goblins.
He chose to let his fury go wild and sprawled through the goblins, dealing his damage to all of then and killing a lot of them (a horde of goblins minus a lot of goblins is still a horde though...). In the process, he lost grip of his sword and flung it down a slope.
And one thing should be clear to everybody after the first miss on a Barbarian's mighy cleave: being in the middle of a goblin horde with your sword flung to the other side of the cave is a glorious opportunity to satisfy some Barbarian appetites!
Best Answer
Stunt Dice
This comes from Exalted. Give them a bonus of +1 to +3 dice when they're:
Do not give the bonus for actions that they repeat over and over, or that are conventional approaches to the problem. Feel out how many dice to give by how impressive the description is. +1 is "yeah that's pretty cool". +3 is "OMG that's amazing, I never thought of that before"
This has worked at my table in a hybrid ruleset WoD game.
Avoid Punishing Creative Descriptions
When using this system, avoid punishing creative descriptions. For example, you may be tempted, when receiving the narration "I sidestep the ghoul's attack" to enforce a Dodge roll to see if the character can pull it off. Don't. Make the full paradigm shift to cinematic if you're gonna go cinematic, and avoid excessive simulationism. If their actions are going to be punished with additional rolls that A) could fail and B) slow down the game, players will tend not to describe anything you could latch onto as getting them in trouble.
This necessitates a distinction between what is flavor and what is mechanical. In the aforementioned case, if a player's main intent is to Dodge, yes, do the Dodge roll. If a player's main intent is to whack the ghoul, the Dodge isn't necessary. The flavor enters the fiction, but does not engage the mechanic. Intent is key.