RE: (Replacement) key fobs
It's called a "Smart Key" because it was pretty smart of Mercedes to come up with a $5 part they can get $250 for, especially if it's likely to be lost, misplaced, crushed or dropped.
I've not had any luck in finding a reasonable source for replacements, and yes, eBay fobs seem to be a dead-end. I hope I'm so obviously wrong that it'll flush out someone to make me look stupid. Please, don't throw me in that briar patch.
I've not run across information to swap out the electronics. I'd get two eBay duds so I'd have one to research on before starting into a working fob of solid gold. Er, working key, that is.
Mercedes do some things that are.... well, making the "soulless" Japanese products appear to at least have some regard for human users.
Cracking cheese, Gromit~