The reapers weren't evil, they were amoral. They did what they did because it was a means to an end. They are no more evil than a forest fire, an asteroid strike or a supernova. Indeed, all those things can bring great destruction and death, but can also bring renewal and the promise of new life, just as the reapers did. I still destroyed the buggers though.
I have actually picked the other options--synthesis for my full paragon sentinel & control for my *barely* paragade infiltrator. Partly because it suits their personalities (what they would choose) but it's mostly for the sake of variety and I have the tiniest lingering hope that the legend savefile will be worth holding onto for future games....and because seeing EDI hug Traynor is hot.
I don't know, I just don't see it being the Batarians. They're like humans, but farther back, so why wouldn't it just have been humans from the start.
My reasoning was just process of elimination. We know they didn't focus on humans until *after* ME1, so they must have had someone else in mind. From what Harbinger says, Asari, Turians, Salarians, Drell, Krogan, Quarians and Geth (obviously!) aren't suitable for a variety of reasons. That leaves Volus, Hanar, Elcor, Vorcha and Batarians. Of those, Batarians appear to be the only ones even vaguely close to humans. Granted, there are in theory dozens of other minor independent races out in the terminus systems, but they would seem to be unlikely candidates.