1. She may always be nippy or she may mellow out with regular handling, it really depends on the snake. She also may be snappy because she's in a new place and is stressed out about her new place. Give her a week or two to adjust with no handling at all, only go in to change water and spot clean if needed. My female has a hide she occasionally uses so I would suggest adding one if you don't have one already, can't tell from the pics.
2. I would start with small-medium rats, mice are a bit small for a 4-5ft carpet.
3. Size varies but she will grow for the rest of her life, growth will slow as she gets older. My female is about 13 years old and she is roughly 7.5 feet.
4. A snake thats 4-5ft has shed more than once, maybe they meant she has only shed once since they had her?
As for handling goes I would wait a while then try getting her out after she has eaten a few times and settled down.
I assume you've done your husbandry research, but if you are unsure of things I'd look for some caresheets

or ask more questions here, if you can't find what you are looking for.