Again, to avoid being flamed, I want to be very careful to talk about my experience in the pursuit of Z fares - and not about the "rules" per se. I don't profess extensive knowledge of ticketing rules, but I have bought 50+ int'l Z's in the past 10 years. The company pays for "C". If Z fares are available I have to take them, if not I can take C's/D's - I know a good thing when I see it and so I don't wait to the last minute to ticket to "make sure" I get a C or D, I make an honest effort to plan ahead and search for Z's - I'm have success a little less than half the time.

Seatcounter: You say "seatcounter shows Z availability". Since you can't put your entire routing into seatcounter I assume you are looking leg by led and since you see seats in the Z bucket MUC-OSL you are amazed that they don't fare out. Actually, this is no surprise at all, just because you see Z fares on each individual leg DOES NOT mean that an itinerary constructed of those legs will show up as Z - that Z fare MUC-OSL might only be available for example in conjunction with a Z fare NRT-MUC (you found just such a fare in your SFO-FRA-MUC routing . . . a connecting Z fare that didn't exist either on a non-stop SFO-MUC routing or on a non-stop SFO-FRA routing).

Frankly, the more complicated (and in your case specific) you make your itin, the less likely you are to find a Z.

ITA is a fabulous tool for keeping you from banging your head against the wall in the pursuit of an impossible fare. ITA would tell you in an instant whether there are any Z fares possible on your routing on a given day (or even in a given month). Frankly, I couldn't live without it.

No doubt others will say it's possible to find a combination Z / D / C ticket. Frankly, I don't think I've ever personally managed to ticket one. A C one way a D the other yes, many times. But a Z with a C or D, never than I can recall.