In fact it is probably unwise to apply current to the digital pins if they are not powered in some way.
I don't WANT the other arduino powered.
Then don't apply signals to them that is probably what knackered your Nano. On the other hand if you WANT what you WANT how about re inventing the laws of physics, preferably in some other universe than here.
You are the one asking for help, I think you need a change in attitude to start with.
That statement:-
Also, the Arduino seems to have some weird logical rules,
Is total bull and is result of you not understanding what you are doing.