"I'm fine with the concept you have in mind for the momentum changes too. Just keep in mind that the most efficient option will almost always be to simply spam the highest potency per momentum ability or GCD during each 5 risk stack window as much as possible, regardless of what that is. A combo potency on the saps/chains could fix this if they're oGCD, but feels a little counter-intuitive putting a combo chain on an oGCD."

I dont think this would actually be possible.
If you have 100 momentum at max, then you spend 20 on Temper, neccesary in order to get to 5 stacks, then you have roughly 7s give or take depending on where you hit it, to finish your combo and execute a Pain skill for 50 momentum.
You simply dont have enough momentum to spam it, and if you did manage to get the right combos in during that 7~9s to regain 4x3 momentum plus a bonus 10, to get you back up to 100, you wouldnt have enough time to actually execute 2x Crimson Pain skills before you dropped back to 4 stacks, if they were on the GCD.
Perhaps 2x Phantom Pains, or one of each, but as an AoE skill I'm not really against that.