Mate, I'm struggling to see what you want us to say. It's pretty clear that in your own mind you've said, it's either this house, or I'm moving back in with my old flatmates who have made me miserable this year... If you've given yourself this ultimatum, then, Yes, of course you're going to be clutching at straws... I'm not going to talk about whether or not I think you're going to get this room, because I'm not a mind reader nor a mystic.
what I can do though is assure you that these aren't your only options.
I can guarantee that there will be other people trying to fill places in their houses. I know you've already dismissed this as an option, because you "wouldn't feel comfortable" doing that, but please ask yourself this question:
If you don't get the room in this house, are you going to be "comfortable" going back to live with your old flatmates?
From everything you've said so far, the answer is no. From everything you've said so far, pretty much anything would make your life a little bit happier if you didn't have to live with them. Do yourself a favour and don't limit your options because you're unsure of what else is out there. The probabilities lie in your favour of finding somewhere else where you will be happier....
Answer ads, and meet up with all of the people you would be considering moving in with. you never know, you might find a whole new bunch of people that you really get along with.
I'm sorry if this sounds harsh, but it looks to me like you've found "the perfect solution" (if it weren't for this other girl) and aren't willing to consider anything else if that doesn't work out.
Hun, I know how difficult it is to live with people who get you down. I've been in a position where it got so bad I had to sneak in and out of the house so no one knew if I was in or not. It got so bad once, I booked into a B&B for a week... But, when I was in that position, I knew the absolute worst thing for me to do would be to move back in with them. Eventually after some looking, somewhere worked out. It wasn't the most ideal solution, but I landed with my feet on my ground and I've been happier with my living situation this year than I was last year.
Please do a bit more searching around - it'll be to your benefit. And, do it as soon as possible to open your mind to the options to help you stress about it a little less.
Take care,
Cxx