Playing spirit ghouls in decline
(1) You are correct - if you Decline your active race, that's all you do in that turn.  There's no window of opportunity for conquering at all.

(2) I believe Declined Ghouls operate as a special case: normally only one token of a Declined race stays in a region, so it's very obvious what happens when that region is conquered (the token is obliterated).  Declined Ghouls however have all the characteristics of an active race, so I'd say that, as long as you still have Ghoul-occupied territories on the map, you can redeploy them in just the same way as you would your active race.

(3) A Spirit race never leaves the map, even when in Decline; Ghouls can still carry out conquests even while in Decline.  I don't any real contradictions going on here: as long as your Spirit Ghouls stay on the map, and it may be a very long time, they can still conquer territories and generally behave much like an active race.  Note however that you can't do some Conquering with your Spirit Ghouls and also put another race into Decline in the same turn.  It's one or the other out of Conquering or Declining on any given turn, that really is the golden rule! (unless the active race you choose to put into decline has the "Stout" power - In this case you could use both the Spirit Ghouls as well as your active race to attack, and after scoring still be able to put the active "Stout" race into decline, therefore reaping the benefits of both special powers)