1. Nope. One custom UI per game installation (meaning you would need to have the game installed in 3 different directories for 3 different characters. Although, you could try a variation of the idea posted
here (different .bat files for each character).
2. Some custom UIs may not function as they should after any updates SOE pushes out (usually only the major updates). Part of the joys of playing with custom UIs is having to wait for the author of any mods you use to update their code (then, of course, download new files they upload) in order for their work to play nice with the changes SOE rolls out.