I inferred that Grindelwald used the stone to create them, but I believe it wasn't directly mentioned so you could be right. I'll pay closer attention to it next time I read the book. The echo of the dead people I viewed it as just one of its powers not the only one, otherwise I don't see much use for it, and it certainly wouldn't be dangerous.
Dumbeldore was just mentioning the plans the 2 of them had for after they obtained them, he never got his hands on the stone. Marvolo had it during that time i believe.
[quote:2821f]There was another detail that kind of nagged at me concerning the whole 'true owner of wands' thing. Does the wand have no problem going back to a previous owner who was defeated? It certainly appeared so. Take the DA for example, they practiced stunning and disarming each other so many times, are they all 'true owners' of each others wands at that point?