I didn't really mean to suggest that Melian had all this in mind when she revealed herself to Thingol. Rather, that she loved him and intended to become his Queen (if that title is appropriate), and all that would ensue therefrom; and it was Eru who knew just what that would all be. I really meant that all those things were results of a once-in-time choice. Sort of like real life, eh?
But I'm not getting answers on the newly stated Dark question:
If Sauron didn't need to be incarnate in order to subcreate, own, possess, ruin, torture, and make a Ring to do the same, why become incarnate at all? Was it something that Eru required in order for Valar to be part of the History of Arda? Were there ever any really non-incarnate Valar in Arda? Ulmo does not fit this description, as he was incarnate as Water rather than as Human(oid).