I've played with the GamePad while charging plenty of times and it doesn't seem to have any charging problems whatsoever so it's not something I'd worry about.
I do it all the time so no
And you've been testing the battery's effective capacity over time? Can you please show us a graph of that data? How have you been doing the experiment?
I don't see why I should no one else has
Yea no one has, that's the point. So we can't really answer the question, and my answer wins.