My understanding from your followup comments is that the player is not proposing to actually gain Druid abilities through this arrangement, but to have (a) opportunity to portray the warlock as two different personalities, and (b) ability to use Astral Projection.
I think there are possibly three different questions here.
- What is wrong with this, from a fictional/narrative perspective?
This is what your points (1) and (2) seem to focus on. My opinion is this is all subjective, and it is up to you, the player, and the group to decide what fits your campaign. Personally, I think your two points make sense, but this is all about fictional positioning, so there is no definitive right or wrong.
- What is wrong with this, from a rules/balance perspective?
This is what your point 3 seems to focus on. The player is apparently proposing that they control a second character, who can astral-project at will. This is certainly well above the normal power level for level 7 PCs. From a strict rules perspective, this is totally inappropriate. From a balance perspective, it is highly suspect.
- What is wrong with this entire situation?
You have stated you have just started both playing and DMing. You say the player is a min/maxer which implies to me they are more experienced with the game.
A cynical interpretation is that the player is trying to take advantage of your new-DM status to gain abilities that they know quite well are overpowered. Or perhaps they just are trying to express an idea they thought was cool within the existing game mechanics.
Either way, my suggestion is that you should respond along these lines:
This is my first time running a game, and I am not prepared to gauge
all the ramifications of allowing something not covered by the
standard rules. I ask that you limit your character's abilities to
those covered by the standard rules. You are welcome to role-play
your character however you like -- if they change personalities daily,
you can portray that however you like -- but in terms of mechanics,
they must function as a single character of the given level as defined
in the rules.
For a more experienced GM, another reasonable response might be:
OK, I think I understand what you are going for, and I am prepared to
allow it -- except that the sibling within the pendant will be an NPC
controlled by me.
A player trying to gain extra powers will likely balk at this proposal. One who is simply interested in the narrative possibilities will at least consider it.