The MB CPO warranty might cover it if it's the electric adjustable version, but not the manual one, as there is no warranty on the seats, but there is warranty on all electrical switches, harnesses, motors, heating elements (seats, steering), etc, etc.
The back comes off the seat quite readily - two Phillips screws, one near the bottom each side - and then lift up. You can see all the guts in your seat - you'll be surprised how little is in there.
Should be able to reach the lumbar support and find out why it fell in - most likely a screw came out. Use the Mercedes EPC (electronic parts catalog) to examine what parts are available, order the new one, and you should be good as new.