let us know how your test comes out. I've used Mustard, Apple Cider Vinager, ACV/Apple juice mix and other concoctions and haven't really noticed a difference. If anything, I think the bark is better with mustard.
You will get more proficient at butts. The key, in my opinion, is patience. It's easier to "hold" them if they get done early than it is to rush them at the end. A lot of folks brine them here. I don't. It's considered sacrilegious where I learned to bbq in KC. But I'm sure they come out very juicy. The best butts I've cooked were at low temps (205-220 degrees) for a long long time.