We had terrible problems with one of our cats 7 years ago weeing on our beds or the sofas , and after 2 years of trying to help her we decided that our home had become too stressful for her what with builders and teenage childrn and their music and friends ..... We tried all sorts of things from the felliway spray, antidepressants .... Finally we decided that we were actually being unkind to her making her live in an environment she was clearly not happy in and managed to rehome her.
A couple of years later we aquired another cat who was clearly showing signs of being bullied by one of the original cats, but as he got older became the more dominant cat.
We suddenly realised that although we had a cat flap open all the time and a litter tray downstairs, the older cat was guarding routes to both and had clearly done so with the one we had rehomed
Easy answer was to have had more litter trays about the house and then poor Hettie would have still been living here.
As far as the bannisters is concerned- why not put slippy towels etc on the surface so the cat is reluctant to jump up there in the first place???- just an idea!?
Good luck to you!!