Well, Dean, I think you've summed it up pretty well the way it is. If you want to stealth camp in Germany (which people do manage to do, I am told), you need to be very discreet and not be too upset when some hunter who thinks he owns the forest (well, he sort of does, at least more than a measly stealth camper, esp. if his club has leased areas of it for hunting) tells you to get lost. A bit of charm and a humble attitude don't hurt, either (as in "Oh, I'm very sorry; didn't know the rule, being a dumb American, but hope I've not disturbed anything...and by the way, I'd really like to move on and stay another night elsewhere, if you don't mind, ; after all, it's such a beautiful place, and I promise to respect it.") The whole idea of having to justify onesself for just enjoying nature up close - in a very responsible, leave-no-traces way - doesn't appeal to me at all, which is why I've only stealth camped in Germany on three occasions so far (didn't get "caught", but felt like a criminal). I just don't want to have my outdoor experience ruined by having to hide. This even makes it harder than usual to find a spot to pitch your tent - a spot that no one is likely to see, but without ending up smack dab under some sappy, dripping pine tree.
I've been living in Germany since 1982, and have learned that it's better to not ask too many questions or else you'll get all kinds of dumb answers - some justified, many not. Whether or not any given person decides that what you're doing is OK (or whether they liked the tone in your voice or not), you'll get answer A or maybe B. It's the luck of the draw. There ARE a lot of rules (including unwritten ones) in German culture, and it seems to be a sort of hobby to interpret them in one's own way, sometimes bending the law without batting an eye in your favor, but more often sticking to laws that really don't apply, or that shouldn't really apply to a zero-impact camper.
I just scanned a few German forums and found people reporting on many good and a few not-so-good experiences. The concensus seems to be that yes, you are SUPPOSED to get permission first, but this can be very difficult, since - as you point out - it's hard to figure out who's in charge of what. So it's apparently better to just try doing one's inevasive thing and bear the consequences, if any. If you can put up with the suspense, that is. It seems that things work out fine maybe 90% of the time, but I also read about a guy who was camping outside of the forest on a clearing, but in an area off the beaten path, so it was unlikely that he had been easily visible. Still, someone had seen him and gone to the trouble of alerting the police, who greeted him in morning with a couple of polite questions like "How long do you intend to stay here" (moving right on today...) and "Isn't it rather cold tenting here?", and then they left him alone. The point is this: In Germany, there are folks who will get all excited (and actually scream at you) if you are just using the bike path on the wrong side of the road (!). However, tose might well be the same law-abiding individuals that won't think twice about pushing ahead of you in the so-called "line" at the bakery. Here I get the feeling that "law is in the eyes of the beholder". Most people speed when driving, with few radar traps to stop them. It's a mentality thing. There's a famous saying: "Wo kein Kläger ist, ist auch kein Richter." This translates roughly as this: "If there's no prosecutor around, there's no need to fear the judge."
As for me, I enjoy the great outdoors in Germany a lot in day hikes, and am longing for a trip to the UK or Scandinavia to satisfy my more wild urges. Sad but true. (And now I HOPE a zillion others will post that they have had lots of great times stealth camping in Germany. Please, prove me wrong!)