In regard to your questions:
1) Is it possible that my valve guide seals have gotten brittle but that my valve guides are fine?
Yes, this is possible, but with 76k the guides are not far behind, usually need to be done at about 100 to 150k. The newer guides are made of a stronger material than the originals.
2) Can a valve guide with play really cause a valve guide seal to go bad?
Yes, but the wear would have to be significant. Valve stem seals usually succumb to heat cycling and get brittle and are unable to seal properly.
3) Is the first mechanic just trying to sell me an unnecessary valve job?
No, but I wouldn't totally dismiss his advice. $1500 is about average for this type of work. I would try a compression test, if that's OK, then try just the seals. You may be able to buy some time, but the guides will have to be done eventually.
4) In what years did MB modify the valve seals and/or guides in the M103 engines? I read somewhere that 1986 and 1987 engines had valve guide seals made of one material and that 1988 and newer seals were made of more durable viton. I also have been told that the valve guides themselves to the 1991 and 1992 engines were stronger than in previous years.
I believe that in late 1989 they changed the material for the guides. Don't know about the seals.
5) Do M104 engines have weak valve guides and seals as well? (Since the head is totally different, I didn't know whether these engines shared the same valve guides and seals.
Don't know this one either. Maybe Benzmac has the answer. I beleive the valves in the 104 were a different size. Anyway you've got a 103, so it really doesn't matter. Good luck...
1987 BMW 325