^^ +1 for the suspenders, sounds like it will fix your back freeze problem. Also, many companies such as burton offer jacket to pant linking, which zips the pant to the jacket almost completely to make sort of a one piece suit.
686, DC, analog, nomis are some brands off the top of my head that offer at least 1 or 2 suspender pants.
As far as fit, columbia as more of a ski brand will most likely have tighter fitting clothes, but in the snowboard world most everything will be looser fitting, and when it is slimmer it will usually say "slim fit." But from experience 686 pants are very roomy, and so is burton's "team relaxed fit", both of which are pretty solid clothing manufacturers.