This is not balanced.

Okay, I don't usually do "is this balanced" questions, because I don't really know what 'balanced' is. But, to paraphrase Potter Stewart, "I know it when I see it, and this ain't it."

You've obviously tried to balance it, and that's laudable. And there may be room to make this concept balanced. But it's not there yet.

1. First-off, let's note that it's piecewise-equal to each of the weapons it's based off of. That is, in "dagger mode" it's as good as normal daggers, in "spear mode" it's as good as a normal spear, &c. for quarterstaff and javelin and the club-and-dagger variant.¹ This hits configurations 1 and 3-6.

   Then we get to configuration 2: I don't know what-exactly this is, but it does the damage of either a Greataxe or Greatsword while being half the weight of each and bearing the keywords Finesse and (!) Versatile, rather than the Heavy and Two-handed of the Great weapons. That extra ! is in there because it'd be the only Finesse and Versatile weapon other than sun blade, a rare.²

   [Thanks to SirTechSpec] Configuration #2 also gets into murky water with your druid's weapon proficiencies; a druid's got scimitars but not other swords, and this configuration looks a lot more like a sword or even a polearm than it does a scimitar.

   [Thanks to Eric] Another easily-overlooked problem is that your gnome is not capable of handling heavy weapons; you've created a non-heavy weapon that does the same damage as heavy weapons, so you've undercut this bit of racial balancing that the designers built in.

   So you've got something that's equal to existing weapons in five configurations, and is superior in all ways to two other existing ones in the sixth. At it's core, this design is unbalanced.

2. I see you tried to balance it through the action economy. Unfortunately, you didn't. Compare your tinkerer using this to someone carrying the five comparable weapons. (I'm ignoring configuration 2.) Your gnome has to spend an action (or less) to switch weapons.

   But our comparison-character would have to burn an action to achieve the same thing: stowing one weapon might be one of the "free" actions tabulated on PHB190, but "Use an Object" (PHB193) would come into play when drawing the second.

3. Weight: your whole thing weighs in at 4lb., I believe? Someone carrying all the implied weapons weighs in at 12lb. (I called your staff-sword a longsword, for weight purposes.)

4. I suppose some might argue that you can be disarmed of five weapons at once, which is the one drawback I can see of the whole design. But I think it's a hard row to hoe, calling that drawback equal to shedding 8lb. and gaining a Versatile, Finesse, one-handed, non-heavy Great[sword|axe].

There's another important way in which this isn't balanced.

All of the above addresses "in-game" balance: how will this character's capabilities compare to those of other characters. But I'd actually be more concerned about "table-balance." You've chosen to play a gnome, which has as one of its bits that you can design a little trinket, which probably doesn't come into play very often. But you're trying to bring that to an item that you're like to use during every round of combat.

To the extent that I can see combats at your table including any time spent on your character swapping configurations or you [the player] discussing with your table-mates which configuration would be best in this fight, you're unbalancing the table. You're creating a "weapon-picking" game that only you get to play, and it comes at the expense of your mates.

Wizards, particularly, and spellcasters, generally, also have to juggle this: they've often got many more options than martial characters. But their classes are build with forty years' experience balancing this. You're proposing a character have unique equipment with all this complexity, but no drawback. It's jut not really "playing nice" with your table.

^{1 - And I have to note here, I really like that bit distinguishing the javelin-mode and the spear-mode. You've done some nice fantasy-tinkerer-engineering here. Let nothing I say above detract from the interesting ideas and obvious thought you've put into this!}

^{2 - a wise (mini)man once told me that options=power. Versatile + finesse is giving lots of options, compared to other weapons.}

Darkness (usually) cancels out all Advantage and Disadvantage effects for people affected

This is because of two basic rules that affect how combat resolves.

For the person outside the Darkness:

• They cannot see the target (even if they know their location, which they do unless the target uses the Hide action), so they have Disadvantage on their Attack Rolls
• Their target cannot see them, so they have Advantage on their Attack Rolls

This is symmetric for the person inside the Darkness as well: they cannot see anyone they attack, and anyone they attack cannot see them either.

The semantics of these rules are found in the Unseen Attackers paragraph of the Making an Attack section of Chapter 9 of the Player's Handbook:

Combatants often try to escape their foes' notice by hiding, casting the invisibility spell, or lurking in darkness.

When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see. If the target isn't in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target's location correctly.

When a creature can't see you, you have advantage on attack rolls against it.

If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses.

Since Advantage and Disadvantage cancel each other, and you don't consider stacked effects (i.e. 2 Advantage sources + 1 Disadvantage source doesn't result in net Advantage), Darkness almost always results in all advantage + disadvantage being negated for all affected creatures.

Shadow Sorcerers, Devil's Sight, and True Sight break this Symmetry

There are three features in 5th edition D&D that alter this situation.

• When Shadow Sorcerers cast the Darkness spell using their own Sorcery Points, they are explicitly permitted to see through their own Darkness spell
• Any creature that has the feature Devil's Sight (most high CR Devils, and Warlocks that take the Devil's Sight Eldritch Invocation) can treat all Darkness, even magical Darkness, as Dim Light (which has no effect on Attack Rolls)
• Any creature that has the feature True Sight (some creatures, as well as anyone who has the Truesight spell cast on them) may ignore Magical Darkness entirely, as though it's not there

In each of these situations, the Unseen Attackers rule comes into play, where the creatures that have at least one of these features will have advantage against any creatures that don't when attacking them, and said target creatures when attacking will have Disadvantage against their better-sighted assailants.

So what's the point?

Well, for starters, it does negate Advantage that your opponents might have. That's one good use.

Secondly, it makes it easy to Hide. Hideing requires some kind of cover, or some ability to stop the enemy from seeing you. So long as that dagger is out, the creature trying to evade the enemy can Hide as an action whenever they want, and if their Dexterity(Stealth) roll is high enough, their location becomes unknown to their enemy (though known to be somewhere inside the Darkness bubble), which will make it much harder to hit them, even without Disadvantage on their attack rolls.

^{Resolving the unknown location, mechanically, is DM fiat; generally, the DM makes creatures target random points within the bubble, and if they're more than 5' from the creature, they're not close enough to hit them. Otherwise, they use their Action to use the Search action, and if they roll high enough, they're able to know the location of their target}

Another important use of Darkness is to prevent spellcasting that requires visible targets. The vast majority of spells, especially single-target spells, require visibility of the target in question; obscuring vision with Darkness makes such spells unusable.