For competitive trail riding I would pass a walker by again. I'd go for a Mountain horse. They are energizer bunnies on a trail, and I have seen them run circles around a walker for long distance trail energy.
I just think you really need to go out and ride a few different walkers. I don't find them comfortable, especially when walking. I have had two, both had a very long overstride which can throw you back and forth in the saddle, and trust me that wears on your back flex as much as bouncing up and down. When they went into gait they were both smooth as silk. I really love walkers, I have learned I just need to do it from afar. You will also have to watch the bloodlines closely, as there are some that are very hot with an overabundance of go go go and are very hard to keep at a walk since you are talking of sharing with your mom.
And don't say anything about trotting to most gaited people. They call that the biggest sin as most will after a time start trotting all the time (it is much easier and requires less muscle) or become horribly pacey. Neither is anything comfortable. I also had a Kentucky Mountain horse that was allowed to trot and she never would go back into a gait, so that kind of defeated the purpose and in this market made her a very hard horse to rehome. I finally did but she was sold more as a broodmare more than a riding horse since she didn't gait anymore. To put it bluntly I pretty much gave her away.