[Calahan]

Testing I did at some point (maybe a year or so ago) showed that enemy prophets that got enslaved and then GoR-ed, spread dominion for their original nation. It also seems to prevent the original nation declaring a new prophet. (which makes sense if the old prophet is still alive and spreading dom). I have no idea if this changed in a patch at some point, but I haven't seen any patch notes indicating it has. Easy enough for anyone to test though if someone is really that interested.

I have no idea about R'yleh prophets that occur from the insanity though. (R'yleh has enough fangirls that I'm sure someone has tested it and knows the answer by now)