Hey --
So did you delete all the attachments on the fascia? apart from just the border ones?
Back leg issues could be self intersections on the muscles and or bones. Remember if there's any self-intersections at any point during the simulation, you will run into issues with the fascia collisions.
You could make a script that uses ziva's self intersection detection tool on every frame, so that you can evaluate the muscles bake before running the fascia sim and make sure it's self-intersection free.
Also just double checking you're not using that zPolyCombined mesh any more, right? Just collide with the individual objects.
Your rest-scale should be in the range of 0.80 - 0.9 I think. 0.99 just means it's going to shrink 1%. That won't be enough for the fascia to be taut over the underlying objects.
Pressure looks low to me as well. Try 100.
Fascia model looks good 🙂