Dad is starting to see the light, but he's stubborn.  He did the mirror
thing.  Saw no fuel tank.  I told him that's because it's slapped on the
backside of the rear seat backrest.

Old age and freedom make for an interesting combination!

He agreed to call a Mercedes dealership on Monday.  He'll also try a
locksmith...but I told him that a locksmith is almost certainly unable to
produce a key from the VIN.  He also wants to inquire with his regular
Mercedes mechanic...who will undoubtedly tell him exactly what you've said.

I commented to Dad that the absence of his trunk lock vacuum actuator has
also contributed to his predicament.  If that long-dead item were
functional, then his trunk would be unlocked whenever he used his door key
to unlock the cabin.  In such a case, he would have been able to open the
trunk merely by pressing the trunk lock with his thumb.

Feel free to affirm or rebut this assumption on my part.

Thanks again.

It's entirely possible the missing key is somewhere else entirely.