I can respond to at least your second question right now: I can power the Blueretro from USB while on the console that's how I know it is functioning properly when powered properly (my original post wasn't clear enough I guess).
I'll test the voltage while powered on as soon as I can.
What I’ve measured so far:
There are some active components inside the adapter itself, I guess this could explain the drop 0.2V drop.
So is it sagging?