My bridesmaid bailed... advice?

Asking 9 months in advance is not asking too early, FFS.

Anyway, invite her to the wedding as a courtesy. If you'd really like to mend whatever friendship is left with this person, try to set up a time to talk and address y'all's situation. If she declines the invite to the wedding, then that's a pretty good sign that the friendship is over, OP.

If a bridesmaid says, "NO," Does she have to give the proposal box back?

Don't worry about it! At the end of the day, you will be getting married whether you have 9 bridesmaids standing next to you, or 1.

Sorry to hear that. Couldn't really give advice as I don't know the full story/ what's going on in her life or whatever. As for the wedding, I'd just move on. Have one less bridesmaid. Nbd.

I had a bm bail 4 months before the wedding, luckily, another close friend of ours offered to step in without me even asking. I was reluctant at first but she seemed really excited to be a part so it worked out in the end.

Sorry that happened. A lot can happen between now and next May. Maybe she's going through something. Give her a little time and try to reach out to her. I would still invite her at this point.

Don't replace her. Invite her as a guest. To me that is the most you can do. You can also continue to reach out, but if she ignores you every time, you may want to rethink that. It could end up making you feel really shitty and even more stressed out. It hard OP, but you are not the first bride to go through this.