I'll do my best to answer your questions:
1. 181cm. I'd say they ski pretty "true to size". I'm 5'10" 160 lbs. I typically prefer a ski in this category to be at least 180cm. If it has a ton of rocker then closer to 185cm. I was really happy on the 181cm. I personally wouldn't want to go shorter. Longer would be kind of fun to try, but only if I was skiing really open terrain and was mounting them closer to center.
2. It's a different animal than the Enforcer in my opinion. Two sheets of metal in the Enforcer vs the CFX Superfiber and single underfoot Ti plate in the QST give a nod towards the differences. I found the QST to be more playful than the Enforcer. It likes to leave the ground and jump around all nimbly bimbly. The Enforcer feels much damper and has a more power.
3. The QST 99 is pretty similar to the Q98, but I thought it skied much better. It's more stable, but is still easy to make quick turns on. I personally like the shape more on the QST than the Q98. They've taken out some of the "sharp" points to the early tapered tips which makes the ski much smoother and surfier (surfier should be a word.)
You've got two great skis with the Enforcer and the QST 99. If you're more of the hard charger, "let's go make Super G turns down this chopped up bowl at 50 MPH" go with the Enforcer. If you're more of the playful type skier who's always looking for things to jump off, places to make little slashing turns, tight lines that no one else has skied, etc, go with the QST 99. It's still got enough stability for high speed skiing, but it really can't match the Enforcer in terms of power and stability. Contrarily, the Enforcer doesn't come close to the playfulness of the QST.