Tying takes a little practice, and I'm no expert but getting better. Use a helper if that helps, but all I do now is pre-cut strips of string, tie, snip off excess. Tie a bit tighter than you think "you should have to do" so that it doesn't slip apart.
I've never brined; can't answer. I just put some rub seasoning on and go. It's my favorite way to cook whole birds. You can do two at once and see which way is better on the rotisserie. Can't imagine the size of bird should matter, and I never seem to have a choice in stores anyway.
Give it plenty of heat and just let it go. Stop the spinning at about 45 minutes and check bird temp to see how close you're getting.