First of all, we knew that before we even knew Magus' name or that he came from an alternate universe, so it doesn't exactly match the scenario I was alluding to where I have one opinion and then new information makes me change my mind in time to be right when Dan makes the answer official.
Secondly, at one point Pandora said something to the effect of "You need two who are one". I've always assumed that any Dewitchery Diamond duplicate and the person they were duplicated from would work for Magus' resurrection, it was just that Ellen was the only living Dewitchery duplicate alive in the main EGS universe and arranging for another duplicate to be made would have been even more difficult than trying to get Ellen to zap Elliot.
If I found out they had been in a relationship for some time without the readers being informed, I would be highly annoyed. If we actually saw them form a trio I'd have a moment of giddy happiness. Either way, my next reaction would be "Okay, now give us cute scenes of them doing romantic things together."
...Which is all just to say that I'm not interested in the possibility of a Tedd/Grace/Sarah relationship for the speculation, I'm interested in it for what we'd get to see if it happened.
As for the Magus revelation, beyond being disappointed that I guessed wrong, a big part of my disappointment comes from one of the reasons I didn't think Magus was Elliot in the first place - in EGS (or any other story with alternate universes as a prominent element), the idea that a mysterious new character is an alternate of a known character is just too easy and obvious an answer.
It's been implied that Pandora has two mental modes (possibly without a clear boundary between them) - one in which she is a fairly kind human-like individual (and loving mother / widow), and one in which she is uncaring, nearly omniscient, and extremely bored. And until Sister III most of what we've seen of her has been in the latter mode; I suspect it's the state she's spent much of the past few centuries in.
I don't think Pandroa ever helped Magus out of the kindness of her heart; she was bored, and thought that warning Edward Verres about Magus then helping Magus to regain his body would alleviate her boredom.
(That said, I can't see any reason why Immortals couldn't impersonate one another, so your idea is possible.)