That's a tricky one you have there, the butt crack height.

I'd say the results would look better if a JCM kicks in when both thighs are bent to that extreme that lowers or pulls the start of the crease downward from the tension of the flesh. Now I'm saying in theory... the reason it would be a nice theory is because such a JCM is on the center of the figure so you can't simply create the morph, split it, and attach it to both rotations under that condition only. It wouldn't be right if it kicked in when only one thigh bends, the asymmetry would be wrong. It would have to be a strict value operation that applies the morph under the condition of both rotations happening, and only then. Does this make sense?

I've never been able to do this. Multi axis rotations on a single actor JCMs are doable, and I've managed to solve the math between negative and positive simultaneous rotations (I was over thinking the instructions that Nerd gave me on that, as I usually do). Multiple actor rotations single JCM? Center line?  Stumped on that one. I have the strong feeling it's possible though, it just escapes my ability to figure out and apply a formula.

Long story short, it would look better if her glute crease wasn't so long. Obviously this can be done easily by a manual dial fix, but that wouldn't be fancy ;)

Probably the easiest fix there would be raising the glutes higher or tighter, de-emphasizing the length of the crease. Probably the most sensible, easily done with weights or morphs. And more asymmetry "friendly" or correct to the tension of the individual thigh bends.