No, it's a bit weak and too specific
Balance-wise, it's a fairly bland weapon that does some mildly interesting stuff, but requires a great deal of investment without a lot of gain. In the end, you get a weapon that someone else could have just bought. The Dual Wielder feat lets you pull out two weapons, so there's not much difference between using this item compared to just buying a few weapons to swap to.
But strength set aside, that's not my biggest gripe about the item. It's way too specific.
To summon any kind of weapon, you have to have a Pact of a Blade. To utilize these melee weapons, you almost have to be a Hexblade Warlock.
If you take each class and subclass and subsubclass with equal chance, there is an 8% chance someone is playing a warlock, a 33% chance they have a pact weapon, and a 17% chance they are playing a Hexblade. That is a 0.45% chance someone is able to just use this item, without even considering feats, a 1 in 200 chance!
Consider this against the Staff of Defense that has the same rarety, which gives +1 AC to the wielder, and the ability to cast Shield and Mage Armor from the staff if it's in your spell list. Even if you're just holding it, it still provides a benefit. It's still specific while still being able to benefit at least one person in any party. But your item is specific to only you.
I would recommend providing benefit to anyone, but in ways that help you the most. Something like this:
This purple-gray blob can shape itself into any standard weapon once
you attune to it.
Attunement takes an hour of holding and molding in your hands, as you shape it into various points, discs and shafts.
Once it's attuned, it can change its shape to the weapon of your
choice as a bonus action.
When you change the weapon's shape, you may spend a spell slot to split off a mirror copy of it in
your other hand. It lasts for a number of minutes equal to the spell
slot spent. Copies retain their original shape if the primary item changes.
The owner of the item can magically swap the original with a copy without using an action, as long as he's holding one of the weapons he's swapping.
Ammunition can be fired from the weapon. It magically appears, and disappears once it strikes something.
Ammunition or copies of the main weapon are translucent in appearance, and can be dispelled by magic or at will by the owner.
The primary weapon is considered a magical weapon for the sake of resistances, but ammunition and copies of it are not.
This is something I would provide for a group, but they would decide who could best use it. Its strength scales slightly with the slot spent (good for warlocks) and allows you to use different tactics for playing. This makes it quite a bit stronger, but still well within strength levels of the game, since the only strength it provides is:
- The ability to (inefficiently) equip an ally
- The ability to change into any weapon
- The ability to fire unlimited ammunition.
But none of these are game breaking. Everyone should have their own weapon, of their choice, and should be loaded on ammunition anyway. This concept doesn't provide much more than what most adventurers should be able to already do, but it's still pretty cool, and very flavorful for your warlock. It still effectively does everything that you wanted to do, but more, and I'd still say it's a little underpowered.
If you wanted to make it a little more powerful, you could add on something that allowed you to make it a +1 weapon at the cost of spell slots or something, but I think this works as-is.
You could do some gimmicks like making a spear and a copy, throw the primary spear for magic damage and it gets lodged into the monster, turn the copy in your hand into the primary and have the copy stay in the monster, have your fighter use the spear to climb up on something as you change the primary weapon in your hand to a maul to take down a skeleton, etc. But even then, what did you gain strength-wise? A magical spear throw for the cost of 2 bonus actions and a spell slot.
Really cool, but actually fairly mundane numbers-wise.