I'm sorry to hear of the position you find yourself in
You said you can't help but think about the 1% chance that it might not be cancer - and I do understand that. But (and it's a big but...and it comes to you from someone who now has a tripod due to osteo) I'd really be more concerned about the 99% chance that it may very well be cancer and then get that leg and its crappy contents off ASAP. Time is of the absolute essence if it is indeed cancer. I can't stress that enough.
Darcy is a tripod due to osteo and I've thought about this sort of scenario a lot since her diagnosis. In Darcys case, we had no warning of her cancer - she'd had NO symptoms and she just broke her leg whilst walking. She was xrayed within hours and amputated the same day. If we ever had a situation such as the one you are going through, I would (if the dog was a suitable candidate for amputation, because I am aware that not all dogs are) whip that leg off immediately.
I'd rather take a chance and have a cancer free dog on 3 legs than take time to decide and possibly end up with no choices to be made (if you see what I mean).