a) how would seats with the same demographic vote in England/Scotland/Wales (or, for the sake of controversy, the Republic)
That's easy. Most Prots hold their nose and vote FG, with the remainder emigrating or going into terrorism. SF retains much of its support with some bleeding to FF and Labour, and SDLP voters split between FG and Labour. Bottom line, FG completely dominate and SF are solid seconds.
Now someone do a map of Britain with only the Northern Irish Parties standing.
Doesn't Sinn Fein lose most of its reason for existing in a United Ireland?