The iPad, like the iPhone, will indeed turn on automatically if off, and then plugged in. Has always been like that. If it is on and you plug it in, then turn it off, it will stay off.
As far as charging the battery, it makes no difference whether the device is off or on when plugged in, or whether it is actively in use or not when plugged in. Plug it in and unplug it any way you wish, the battery and charging circuits does not care one way or another.
You covered every possibility and I will stop worrying about it. You should get a job writing the User Guide. Lol.