I'm not a big fan of mustard, I'll use it sometimes though.
Couple things
The mustard is just to hold the rub on, some say it helps w/the bark, hasn't for me on anything.........and it makes your hands stink if you don't have rubber gloves.
I've used olive oil, pam, whorchst sauce, soy sauce, beer, and nothing. If you use nothing, allow time for the salt in the rub to moisten and set the rub, then add some more.
Yes rub it all over, incl. the sides. Big hunk of meat(thick) and won't hurt at all, I think the more the better. But watch the salt, depends on who is eating it too.
I'm smoke'n my butts and brisket fat down and really like'n it. So I'll rub the fat cap first, sides and all.....let it "set in", then flip and do the flat/no fat cap side last. That way you just pick it up and straight to the smoker. Allow an hour to do all this, won't hurt and even helps.
Long story short, too much beer I guess, rub it all over real good. Use mustard, not much needed, or whorchst or what ever to "hold' the rub.
Good luck
If you have more questions I won't be so long.......if your lucky someone else will give their opinion.........2 heads are better than one