A question about disembodied monsters and blinding spells on them

Immediate actions are defined as follows:

Immediate Action: Much like a swift action, an immediate action consumes a very small amount of time, but represents a larger expenditure of effort and energy than a free action. However, unlike a swift action, an immediate action can be performed at any time—even if it’s not your turn. Using an immediate action on your turn is the same as using a swift action, and counts as your swift action for that turn. You cannot use another immediate action or a swift action until after your next turn if you have used an immediate action when it is not currently your turn. You also cannot use an immediate action if you are currently flat-footed.

Immediate Actions start in the order they are announced. In your example Wizard A starts casting Apocalypse from the Sky, Wizard B cast Celerity and then combust, then Wizard A casts Celerity and counterspell.

This would then resolve as the counterspell countering the combust, and then the Apocalypse coming.

So in answering your numbered questions:

1) Yes

2) Once Wizard B announces his immediate action it has started and would complete except for the counterspell of Wizard A. Wizard A's counterspell starts a tiny bit later than Wizard B's combust. So no issue there.

3) See 2 above

4) Celerity would cause Wizard A to be dazed (unless he was immune as you have suggested). If not immune I would allow him to complete his Apocalypse spell but I think the RAW answer would be that he could do nothing after the action he gained from the Celerity.

The best way to judge what classes should have access to a spell is to compare it to the kind of spells those classes already have.

Wizard and Sorcerer compulsions tend to be forceful in method and drastic in effect: The examples you gave describe subverting the victim's normal mental process by forcibly inserting a foreign thought, and blatantly stealing control of the victim's mind.

Clerical spells, on the other hand, tend to use more straightforward methods of achieving their ends, and often express the role of clerics as spiritual leaders: Thus, the spells that allow them to speak with authority and be obeyed.

As for why the power of Power X isn't compelling... Well, if you look at the ecology of gods as described in, for example, the Planescape setting, belief is the food of the gods - without it, they lose strength and eventually starve to death, or at least enter a state that is for most practical purposes equivalent to death. A follower who only vaguely follows their patron deity's teachings is still more faithful, and therefore more valuable, than an unbeliever magically compelled to serve. Of course, individual gods might have a thing for magical compulsion, but that's not enough for something to get included in the standard priesthood package.

Oh, and like mxyzplk suggests, there's more game design-y reasons for it, too: if Clerics were good at all forms of magic, they'd be far superior to wizards, even enchanters - and playing an enchanter would be much less interesting.