Wow, that took a while.
The 9.6v mini battery finally arrived in the mail today - charged it up, plugged it in, and the P90 works just fine! Fired off half a mag with no issues whatsoever. I will update the original post with this new info.
So as it turns out, the issue was the 8.4v simply wasn't strong enough.
The 9.6v mini battery sticks out of the battery compartment by about 2-5mm. Therefore, the buttplate can't re-attach unless the plate is modified (there is a big piece of useless plastic on the inside of the butt plate that is in the way of where the wires would go) or a butt stock extension is applied. This is an issue that can easily be fixed, and I'm not too worried about this small problem.
Anyway, I guess that about does it for this thread - thanks, everyone!