I have a mode to ensure you always have the same butler, plus some other butler mods:
http://pearlbhsims.blogspot.com/2017/02/butler-mods.htmlFor the gardening problem, are you actually turning off gardening as one of the butler's tasks and they still do it? I have this problem with taking care of children, even if I turn it off, they still do it.
As for undesirable traits, like simstr0ng said, just turn on testing cheats, put the butler in CAS and give them whatever traits you want. I do this immediately with every new butler because there's no point in messing around with what the game gives you. They're almost ALWAYS mean, kleptomaniac or something undesirable in a butler.