Very weird. Everywhere you look it says they were bought by Sony in 1993, but I remembered reading in magazines at the time when WipEout 64 came out that it would be the last game that they would put out on non Sony hardware. Stating how they were bought up by Sony.
So confusing.
Yes, but the port was done by Spiral House. Not the best version of the game to say the least...
I recall the (public, at least) details pretty well.
Psygnosis still considered themselves somewhat independent, and according to popular rumor at the time, insisted that they be able to release Wipeout first on Saturn, then on N64.
After that, I believe Sony said "no more." Proably didn't sell all that well, or they would have let them continue.
It's also not completely unprecedented. Atari, back in the day, decided that they could make good money on other platforms and lauched the Atarisoft label and made Intellivision, Colecovision, etc ports of their top titles.