Mat Hey Mat! Don't worry. Everyone's here to learn, myself included. So I'll help to answer based on my own opinion, and see if others can chime in too.
Firstly, Tissue does have self-collision. Tricky in some areas and situations, but it does have that capability.
For the 2 images you posted, I would suggest you to firstly build in those folds in your model since those should be there in the default state. Those should come and go in your fat sims if you have the right material settings and enough resolution in your tet mesh. If you find that those folds don't go away even when the pose is stretched, try adding zFibre/zLineOfAction on the fat layer in those areas. Hope that helps.