We are planning on painting woodwork in the house. Do I need to paint all rooms? Can I leave one room, great room, with it’s stain finish?

I am refinishing my garage. I have replaced some drywall. I believe the previous paint was latex…What type of primer do I use for the whole application..oil based or latex? Should I use oil based or latex paint over that?

Pet stains on wall. Put Kilz oil based original sealer over stains. Do I need to seal entire wall? After painting wall will area sealed be noticeable if I do not seal entire wall ?