I am so sorry to hear about your mother. This is a difficult one.
First, since hers is not a typical case, I would want to be sure that she is being seen at one of the very best places for bladder cancer treatment. This would be a medical school or a National Cancer Institute Center of Excellence,
Where is she being seen and where are you located geographically?
Since she is in exceptional health, she should be involved in all discussions about her options. Cystectomy is not a trivial surgery, as you have discovered, but many patients do well with it. Radiation is NOT the usual treatment of choice for bladder cancer..,.,there is often damage to tissue other than the tumor(s) which precludes any other treatment possibilities. Chemotherapy, while definitely not fun, could possibly retard any growth or spread for a while. All of these factors need a caveat "it depends."
Here is a link to the Cleveland Clinic's description of the options for surgery for bladder cancer that many of us have found useful:
my.clevelandclinic.org/health/articles/urinary-reconstruction-and-diversion
In your mother's case, given her age, the ileal conduit would probably be a good choice should she decide to have cystectomy, since it is the simplest and involves less surgical stress.
Again, I would urge a prompt second opinion at a place that specializes in difficult bladder cancer cases if you aren't at one already.
Wishing you all the best
Sara Anne