For what its worth, here's where I am on my issue. I bought a new belt but I have not installed it. I measured the new versus my old and they are so close to the same width I'm just going to run the old for now. I roughed the faces of the drive and driven clutches and tightened the belt. I also sanded the glaze off the belt. This made a significant improvement. Much to my surprise, tightening the belt stopped the creep shown in the first video.
I went for a ride over the weekend and paid attention to how I was driving. I found that on a tight trail the machine is too fast, meaning a little above idle the clutch engages and the machine starts off slow but picks up too much speed (for the type of trail, that is), so back to idle I go and the cycle repeats itself. Obviously a stronger spring is in order to slow things down on the tight trails so I can keep the clutch engaged. Things are looking good!