Maya FBX blend shapes export FIX
It took me probably 30 - 40 hours to fix my issues to export blend shapes to Unity, while using FBX.
The problem is that when you copy your model, copy skin weights, transform attributes etc, somehow it creates history for every blend shape and your original model. And for some reason on FBX + blend shapes export, blend shape connection refers to these old history of meshes/nodes. So of course blend shapes will not look and work as the newest edition. I don't know why this old history is even kept as "delete by history" disconnects connections to these nodes/meshes anyway.
Forgot to mention that you should also delete such nodes fro myour original shape that you are blending.
Anyway solution is to delete these node/ meshes trough hyper-shade.
The file size can decrease dramatically, more scene performance will appear, as Maya does not have to deal with these meshes anymore, and most importantly it fixed my fbx blend shape export problems.
Hope that helps to some of you.