Sim is looking great!
When you say you're going to add the skin fold, do you mean you're going to add this in the muscle pass? I'm not sure I'd recommend that. In fact I had that on the lion but opted to take it out and do this in the fat level.
I can't find any evidence of a muscle in that area in the literature, but happy to be corrected on that.
When you say "How to handle the twists in the muscles, which has different angles at the point of origin and insertion" what limitation are you seeing with your current workflow?
If you take the latissimus for example (human), the muscle also twists right before it attaches to the humerus. You shouldn't need to do anything special in this case as far as I know, so long as the model is correct.
Also can you clarify what you mean by the sartorius changing it's angle during simulation?