There's going to be more to her story than just pregnancy. I'd guess that almost all of the pregnancy content in the game will be somewhat optional and MC can either pull out or use either condoms or pills to prevent girls from getting pregnant. Nobody knows if there may be something related to Diane's story that could make pregnancy mandatory for her at least though. I guess it's fair to say that it likely won't be the best update for those who really dislike that topic.
Yes, there are some. Most of them aren't relevant if you only want to play Roxxy's story and others can be avioded to a degree but they are working on 0.16.1 and it may be a good idea to wait a little if you can.
Yes, old saves won't work. The new content does start on day one though and you won't have to replay any other story (except the very very basic stats grind and a couple of minutes from Jenny's story) to play through it. Else I think there are fitting savegames for you in the savegame topic.