I'm player A. (B is also hitting the Inners)

Scenario 1:

A is hitting the Imperial Inners as their main target. Player B is with me.

Player C hits player B and casts fireball(s), which also hit me. I can't hit him unless I do an attacking move other than murder.

So, I do, say, bash C, after which I do hit C.

>>>Nope... you would have to rescue as C is melee with B, you could do directed attacks at C but cannot melee them with "hit" command

Scenario 2:

A is hitting the Imperial Inners as their main target. Player B is with me.

C enters the room and just stands there spamming area spells. I can't hit him unless I do an attacking move other than murder.

>>>Depends actually, when C area spells they will start melee with someone, if that is you, you can "hit" them

Scenario 3:

A is hitting the Imperial Inners as their main target. Player B is with me.

C hits B. I can't hit him unless I do an attacking move other than murder.

>>>You (A) can do directed attacks against C but not melee without rescue'ing B, or B fleeing.

Scenario 4:

A is hitting the Imperial Inners as their main target. Player B is with me.

C attacks A. Hit C would work immediately.

Is that about right?

>>>Yep


>>>Basically, HIT works to switch between targets you are taking active melee from.
>>>If you are in active combat, (already fighting someone/something) HIT will not bring someone new into combat, you need a directed attack/ area spell for that then you can HIT them after they start melee with you.