mesh coming apart at seams

Hi. Can’t check the file right now but, before you transfer any data to your mesh, you need to remove doubles on it.
- join all meshgroups together;
- in edit mode select everything, Select - Boundary loops;
- mark as seams or sharp;
- select everything again, remove doubles with merge distance set to 0,0001;
- check if uv_1 is ripped in that area where the mesh has a gap in CAS;
- if it is, you will have to re-transfer uv_1. No need to retransfer weights, they are the same now;
- cut and split the mesh again, using seams as a guide.

Checked the file and your uv_1 looks wrong. It should not fit in the uv space.

Unrelated to the problem, but in uv_0 you have uvs in the space meant for shoes

You should always start by studying a similar maxis item, it will prevent 99% of the problems

- clone a lingerie set, export the blend, open, expand the rig, make feet selectable. Select feet, shift-select the lingerie set, join, rename to Reference. Save the blend, close;

- open the blend with your garment, join all meshgroups together, append the reference in the blend;
- select everything with A, remove doubles with merge distance set to 0,0001;

- retransfer uv_1 from the reference. Once you hit Apply, you're done, do not shrink it to fit in the uv space;

- edit uv_0;
- cut and split the mesh again.

You need to read/watch them until the end, at the end you are meant to bring the uvs back to the original. Using a template was how people did uv_1 before Data transfer was introduced in Blender, now there is no reason to do that at all so long the reference is similar to the mesh.

This is an example of a dmap. As you see, it contains only half of the body because the game engine mirrors it to affect the mesh symmetrically. This is how it is for every dmap.
When you shrink the uvs of the whole body to match this image, the morphs start affecting the wrong parts of the body: the front of the body morphs like arms, the back morphs like the front and the back.
The reason it caused you no problem before could be: a). not checking how the mesh morphs; b). vertex painting it with the color that prevent morphing completely; c). Making something small where it would not be noticeable 

I think it might be best if i start completely over with my frankenmeshing and a clean uv_1.