Me and my sister used to play good cop, bad cop - one of us would do the very much needed stamping feet and kicking off, while the other one was more reasonable - it helped keep the nurses on side while getting things changed.
On reading your story, I can see so many similarities, and it tallies with lots of disappointing experiences we've had with one or two consultants in particular. Keep fighting - they've got no business righting your mum off, and it sounds like total bollocks to me.
Actually, my grandfather had a hip replacement a few weeks ago - he has a history of heart trouble and one anaesthetist also told him he wouldn't survive the operation. He is 82 years old, it nearly frightened him to death. My mum and dad stepped in, he had the operation at a better hospital in South Wales with specialist facilities for heart patients, and is recovering nicely, no thanks to the anaesthetist at the first hospital.
Just like the rest of the world, people in the medical profession aren't always right, but unfortunately lots of people will believe what they say as gospel (and you can understand why) and don't look for different advice. I'm glad you've chosen not to accept the first verdict, I think you're absolutely right not to.