I'm not certain if it's against terms of use to change or tweak code (or if the change I want is easily possible) but say I wanted to switch the part of the CB profile wall that says "Write on [X]'s wall" to say "Review [X]" and make it a field that [X] cannot delete, would that be possible (easily)?
For CB ProfileBook Wall tab I'm guessing? Yes, you can modify whatever you like. We don't support core edits though and once core edits are in place we can't provide support until they're reverted. Core edits are also lost on upgrade as well. However as you're staying with CB 1.x you'd be nearly entirely self maintained as our ability to provide support will be limited as we don't support CB 1.x anymore.
Is it a problem for me to plan to remain on 1.9.1 for the long term?
Yes, none of our new releases will work on CB 1.x so you'll no longer receive plugin upgrades and you'll miss out on any new plugins. You'll also miss out on new core features and bug fixes. We also will not be able to provide support beyond suggests as we do not support 1.x anymore.