Hey -- okay I'll get to the cloth stuff in a sec.
I'll go through some feedback on your latest videos first.
For me it's still all pretty stiff. It feels like the muscles might be over constrained or materials set to be too rigid, as I'm not seeing much in the way of secondary animation. If I was to guess I'd say the materials are too rigid because it doesn't seem like the fibers are having much effect on the actual shape of the muscle. I.e. they're buckling rather than compressing:
The muscles in the upper legs especially all seem to move as one block. Rather than individually. The legs do hold up pretty well though when the knee bends which is good.
I would loosen all muscle-muscle attachments, reduce inertial damping if you have it, reduce muscle stiffness if it's higher than default values ( 12 x 103 ) and see if you can get some more performance out of the muscle sim.
I could be wrong but at a glance, it looks like you've made the fat mesh by extruding out the fascia. I would make the fat by starting with the fascia, duplicating it, and then using Wrap3 or Maya's tools to project the fascia duplicate onto the skin. Then you have two layers from which you can make a solid mesh to represent your fat.
The outer layer of the fat will match the contours of the skin perfectly and at this point you can either choose to simply wrap the skin mesh to the fat, or do a coupled cloth solve, to get extra performance..
The cloth solve at the skin level is giving you some cool wrinkling effects. So you probably wanna keep doing that!