Taking your questions in order:
1- They were very common among kohanim and in Yerushalayim.
2- Nothing would stop you from having a mikvah in your home. My local rebbe has not only a mikvah, but a whole shul, in his home. Using one would be a different story as shulchan aruch requires a balanit for the only tvila d’oraisa we still do.
3- AFAIK most of the mikvaos found were larger than the chazon ish’s shiur. Even using the larger seah, 40 is still minimal.