Just to add a bit of info for future battery-changers;I found this out on my 2015 ML350:

YOU CAN DISCONNECT THE MAIN BATTERY FOR AT LEAST 8 HOURS AND NOT LOSE ANY OF THE MYRIAD MEMORY SETTINGS.

I unplugged my engine computer to access the spark plugs, and decided it would be wise to disconnect the battery first.
Just reached under the seat and popped off the plastic cover and the negative terminal - just going that far was really easy.
When I hooked it back up 8 hours later I didn't have to reset windows, seats, or anything.
Only thing that reset was the trip computer - no biggie.
I assume the small auxiliary battery kept everything alive?????

Anyway, appears to be no need to provide external standby power when changing the main battery.
Now, what happens when you change the little battery in the rear is anybody's guess.

Cheers,
DG