AFAIK, the "hold the power button for... a while" trick to hard-reboot doesn't work *at all* with something plugged in (because that trick essentially amounts to disconnecting the battery, and getting power over USB obviously renders than null in terms of forcing a reboot because of power loss... because there's no longer a power loss ^^).
That may or may not explain the results of parts of your experiments
.
That may also not exactly hold true anymore (bonus question: what about the Oasis 1 and its bundled battery, which essentially means it's always plugged in?
).