-- Last Updated: Oct-11-12 11:35 AM EST --
Any of the boats you mentioned would be appropriate to your stated goals and limitations, some more so than others. Comfort might be the determining factor. Wish I could give you personal feedback on all of them, but I've never been in a Karnali. People who own them love them, though, and $750 is a pretty good price for a brand new boat...
Of the models I do have experience with, I think the Remix is by far the best design for river running - it's versatile, forgiving, and comfortable. Your instructor was probably right - if you paddle a Remix and a Mamba back to back, you'll likely know right away whether you prefer the flatter hull or the rounder one. Personally, I like the rounded hull of the Remix.
The Mamba surfs and carves better but is slow in a straight line and has that high-knee posture, while the Nomad is meant primarily for steep creeking (low volume class 4/5 with lots of ledges/waterfalls). It'll be very stable but will tend to wander and wash out the bottom of eddies if you aren't assertive with your strokes. The Remix falls in the middle - jack of all trades, so to speak.
Check to see if there are any paddling clubs near where you live - clubs often sponsor indoor pool sessions during the winter where you can work on your basic strokes and even learn to roll. Then you won't be starting from square one when the weather warms up next season. It'll also help you work on whatever technique or flexibility issue was the cause of your bruised knuckles.
Speaking of next season, stick with the lessons, and once you are feeling good making moves in Ramcat and on the Middle rather than just surviving, look at the Casselman and Cheat Narrows. Both are in between the Middle and Lower in difficulty, and both are neat runs in their own right. And once you are comfortable making moves and not just surviving those runs, you'll be ready to run the Lower at summer levels.
BTW, don't run any of this stuff alone, including the Middle. It's easy to get in over your head when you're just learning, even on class 1.