That actually makes sense and is the only plausible theory of how we reach Andromeda I've seen so far.
Though it does bring up a few questions:
1) How did the reapers get to Andromeda?
2) Why did they build a relay there?
3) Why would Sovereign and Saren lie about the Citadel relay? They said it links to another relay in dark space. Andromeda =/= dark space. It also makes more sense to hide in dark space than to hide in another galaxy unless:
4) Were the reapers harvesting organics in Andromeda too?
I'm glad you like the theory. 
As for your questions:
1) The Reapers have drive cores that has surpassed the limitations we have, such as the need to discharge it. So the Reapers technically have an unlimited range. All they would have to do is want to and they could theoretically reach Andromeda with ease in about 231 years by conventional FTL.
2&4) The Reapers could have built a Citadel and Mass Relay Array there to maximize the efficiency of the harvest of Andromeda, just like they did the Milky Way. The reasons for this could be that it's because their mandate to preserve life was extremely vague since it didn't even exclude their creators from it or because they could see Andromeda as a threat to the Milky Way by seeing the possibility of Andromeda developing intergalactic-capable synthetics that would come to the Milky Way and wipe out life, thus to defend life here they have to harvest life there too.
3) Well, perhaps just like there is a Mass Relay Array to travel around our galaxy, perhaps there is a Mass Relay Array in Dark Space that makes traveling to the other galaxies in our local galactic group like Andromeda easier. We could use the Citadel Relay and wind up in Dark Space, seeing the Milky Way in the background, then use another Dark Space Mass Relay and arrive at Andromeda's Citadel through the Mass Relay built into it.
I like the idea of exploring Andromeda AFTER the endings as well, but it still brings up some of the questions above and a few new ones:
1) How do we manage to get to Andromeda? Even with reaper tech that seems like an impossible undertaking.
2) How will BioWare account for all 3 endings in ME3? There wouldn't be much of a difference between destroy and control, but synthesis would basically turn everyone into glowy cyborgs.
3) Will we all be glowy cyborgs in ME:A if we import a save-file where Shepard chose the synthesis ending?
4) If yes, how will that affect our ME:A playthrough?
1) Well, after the endings we would have years or decades to research and reverse-engineer Reaper drives, either with the ones from Reaper corpses or because the Reapers are allies. Once those are developed, like the Reapers the main obstacle to get to Andromeda is dealt with.
2) For Synthesis, as I said earlier the glow can fade in time so that gameplay-wise it is the same as Control and Destroy, and lore-wise Synthesis was left very vague in terms of the extent of it. All that is said is that with it organics can fully integrate with technology, but that can mean any number of things.
3&4) Probably not, if they have the glow fade after a certain amount of time. So it wouldn't effect anything really. Now if Bioware wants to add that effect layer to all the people from the Milky Way, good for them and hope it works out well.