I did however have someone - without malicious intent as such - who managed to combine several pieces of info he had about me, find several accounts I had and end up finding my email-address so he could email me. That doesn't really qualify as stalking, but it was a wake-up call all the same, and I did some efforts to erase the more careless traces I'd left then.

Your situation seems a lot worse, though, and a lot less easily explicable... if all those accounts were on the same email-address, one has to assume he found that, but that doesn't explain how he knew on which sites to search for accounts. That's quite scary indeed. Just googling shouldn't be remotely enough to get that, although some sites/forums may not be too secure with their information and allow accounts to be found that way, perhaps. Puzzling.