Well, you already know my opinion of the trifecta pad. It's FANTASTIC! Yes the padding is on both sides. The part over the spine is just material, so it doesn't take up spine channel space when the saddle is there.
The shims are about a little thicker than the thinline stuff that is part of the actual pad. There are two shoulder and two rear shims (sold separately of course) You insert them though opening on the pad. You have to kind of fold them in half, insert them, and then work them back flat once they're in. It's easy unless you have really short fat fingers.
As for cleaning, I bought the black one for schooling and the white one with fleece lining just for shows with the hope that it never ever gets dirty so I never have to worry about cleaning it.
You just have to see your distance...you don't have to like it.