Thanks guys,
The page 44 stuff helps a lot. So it seems the answers are:
A1: The stealers and fex may *not* fight the terminators this turn
A2: The terminators may *not* allocate attacks against the stealers and fex this turn
A3: Yes, the sole contacting Terminator may be removed as a casualty, stranding the stealers and fex in the open to get shot to ribbons.
A3a: No to all, as the stealers and fex do not count as engaged and are effectively ignored
The p44 quote specifically mentions that the units are not *engaged*, but does not say that they are not *locked*. If they are locked but not engaged, then the terminators would not be able to consolidate.
I think we have to assume that 'effectively ignored' means that the terminators and stealers are not locked either. Which also means that: if the stealers consolidated into a terminator that was not engaged with the gaunts, then that terminator would be able to either a) consolidate away, if all gaunts die; or b) Pile In to the gaunts, since he would be ignoring the stealers. Either way would strand the stealers out of combat -> shot to ribbons. Actually, depending on how gaunt casualties were removed, you could mess yourself up by disengaging that terminator when you pull the wrong gaunt!
Moral of the story - you can't count on locking units unless they have already fought that turn.
I should have chosen to resolve the Spinegaunt/Termie slaughter before I did the Stealer/Fex/Marine combat.
Hey, at least existing
GW rules were sufficient to sort this one out, I'm impressed! No
FAQ referencing required.