That summarises it absolutely perfectly. 
Absolutely not! Avoid UNOs like the plague. 
Very inconvenient for actual projects.
Indeed.
It is better never to use "Vin" for any purpose in a real project. If playing with a Nano on its own, just perhaps but generally you usually experiment with a bare Nano plugged into a PC which supplies its power through USB so even then that does not make sense.
Whatever do you mean? Given that the switchmode converter is efficient, it will require less than half the current input at 12 V, than is drawn from the 5 V output.
No disadvantages whatsoever. None. Just don't even think of using a UNO. 