This is my very first time doing anything like this, and I thought the pipeline my teacher gave us would work, naturally, but I understand what you're saying. I feel a lot comfortable doing almost anything 3D except for rigging. >.< Here's some screencaps to illustrate my problem. Here is my base mesh (bind pose?)…
I figured something was up with this method. I created a single blendshape with all the heads like you described using the "default" mode, and it's working wonderfully. No weird stretching anymore. I'll post again if any new problems arise. Thank you so much for helping! :)