Hello, to answer your questions:
1 – How can I determine what breed she is? If you can upload some pictures that would help. I can’t help with this one but a lot of forum posters are awesome at giving feedback.
2 – How can I tell if she is overweight? I would say this depends on her breed and her size. Do you know what her diet was. This isn’t very scientific, but if you run your hand down her spine and you can feel each nook and cranny, then she might be thin (again, not a scientific method). If she can’t clean her self (e.g., poopy butt), then she is also likely overweight. For more of an empirical explanation, a vet might be able to tell you.
3 – What are the big patches of extra fur? (not explained well… like double chin, and also on her hind quarters). Not sure what the big patch of fur is on hind quarters, but the extra fur under her chin is a dewlap. Personally, the dewlap is amusing to me and I love it. Females get the dewlap and I believe it gets bigger if they have babies (please correct if i’m wrong).
4 – How long does litter training take? Litter training varies between individual rabbits, but the consensus is that a bunny cannot be truly litter trained unless spayed. So you want to find that out ASAP.
5 – Does she need a hide out? I”m not sure what this means. If by hide out you mean a hidey house or a safe place for her to chill and relax in, then yes. It would be nice for her to have that for fun and even for her own sense of security.
6 – How can i clean the red mulch stains on her bum from the previous owners litter choice? I’m not good at this one so i’ll let others respond.