Weird shadows on the underarm of CC blouse.

Marvelous Designer produces meshes with terrible topology. Compare all the triangles in an MD mesh with the even lines in an EA mesh and you'll see what I mean. The problem you're having with the armpits are the weights. Specifically, b__R_ShoulderTwist__ and b__L_ShoulderTwist__. I can't get them to transfer well and I think it's because of the topology. The topology needs to be improved.

Take your mesh back into MD and turn the tris to quads. According to this you can select all of your mesh, right-click, and choose "Quadrangulate." Tuck the area in the armpit closer to the model's armpit so it's not so loose in that section.

Always model in quads and try to keep your particle distance high. If you have body parts clipping through that you know you'll delete later, don't worry about that.



MD 11 has a retopology tool that you can try.



And Mauvemorn has some good videos on retopology.

Here's a video that you should definitely watch since you're having trouble with the armpits of your mesh.



As for the area that's splitting open, I'm not sure what's causing that.