Really nice result Khalil! Congratulations on the great work.
Some ideas for you.
I can see at the start you're losing a bit of that modeled-in detail as the rest-scale envelope is keyed on. What I suggest is adding another material to the cloth around the delts and in selected areas where you'd like to keep the valleys between the muscles. On that material, double or triple your current pressure value, but keep the rest-scale the same. Paint the material weights to black where you don't want to use this extra pressure. (make sure you have a nice smooth fall-off)
Some notes on getting collisions to behave nicely.
Don't forget that there is the attribute on the zSolver node called collision point spacing.
This will effectively flood all your ziva meshes with extra points that it can use to compute the point-in-volume test for our collisions. This is very helpful for lower resolution meshes that you might be simulating.
Be careful with this attribute. It is in Maya units. So for instance a value of 0.001 will put a point at every square millimeter on the mesh. This will likely be too small for your character. If this value is too low, you'll blow our your RAM capacity because Ziva will have to keep track of too many points. I would start with 0.005. See how that goes.
Secondly, currently with Ziva cloth collisions, you need a lot of substeps. I solved the lion on 8 substeps. Yes it is slow, but the results are worth it.
If you're still finding that collisions are being missed on very narrow meshes, what I suggest is that you create selective sliding attachments in those areas, if they're very important for you to preserve. It's important to make the bone or the muscle the source for these sliding attachments, not the cloth. That is because the source of the sliding attachment is fixed to the vertices of that mesh, and it is the target that is allowed to slide via the attachment. (in this case, the fascia) Making the fascia the source of the sliding attachment will not produce the correct result.
I wouldn't worry about losing wrinkling in the fascia. It looks nice, but really you should aim to get wrinkling out of the fat/skin sim.
Once again, really great results and keep up the excellent work!