Is there a way to decline the offer at B but still keep it as an
Is there some wording that can leave this window open (and without me
being unethical about it?)
I suppose you could admit that you have already accepted an offer that may fall through and ask if they could keep the job open until you know for sure.
Seems unlikely to me, but you never know.
The other option would be to decline the offer from B. Tell them something like "I've already committed to another company, otherwise I would have loved to work with you. Maybe we can get together some time in the future."
Then if company A falls through, get back to company B quickly and ask if the job has been filled yet.
If they liked you enough, and if not too much time has passed, your offer may still be open.
I once was looking for a new job and turned down an offer. I got laid off from my company about a week later. That afternoon I called, and the job was still available, so I accepted. (My original instincts about the job turned out to be correct and the job sucked. I didn't stay long - but that's another story.)