>>368394332
This is what I do, may or may not work for you.
1. Before even modifying the model, select all the UV islands and do "seams from islands"
2. "Pin" the verts that you want to keep in place
3. If you check "live unwrap" and drag the uv's around using G, it'll unwrap itself assuming you set the seams correctly. You can pin more and more verts and try to get the shape right.
Since you've already modified the model, you'll probably need to still make seams but actually rip the parts that are modified and try to re-attach them.