After becoming a mare and buying your name back is there a way to get back into the regular gameplay? Or are you just forever stuck  as their toy and have to restart?

It's meant to be a Game Over. You choose one centaur to spend time a few times with, you can then marry them, and that's it. 

But if you buy a town portal scroll before you are abducted, you can escape. And even farm some money before that