Last I checked a Macguffin was something that was a plot device that had no real point except to push the plot along. If that "Macguffin" is useful to the main story, such as in Fallout 2 making the GECK destroy the Oil Platform, then it is no longer a Macguffin.
In the same vein, a Deus ex Machina is something that comes out of nowhere to fix an insurmountable problem. That would be like the Sun destabilizing for no in-plot reason (If, say, Shepherd detonates an *insert huge weapon here* in the Sun's core to destabilize it, then it wouldn't be a Deus ex Machina, it would be a plot driven answer to a plot driven problem), and destroying the Sol System, and the Reapers with it.
If they introduce something (a plan or weapon or just a general strategy that will lead to one of these) that can defeat the Reapers, and they introduce it before the halfway point of the game, then it's a plot driven answer, and neither a Macguffin nor a Deus ex Machina. If, on the other hand, they introduce a plot device that leads to nothing in and of itself, but leads to the answer; while that would be a Macguffin, it still wouldn't be a Deus ex Machina.
My belief? I have a feeling that the derelict reaper from ME2 will be a major part of ME3. I also have a feeling that the Rachni could feature heavily. That said, I don't think there will be one specific answer. I think that it's going to depend on the choices you made in ME1 and 2. Will there be a way to lose? Probably. The stakes are too high in this one to not have a losing ending.