Also remember that in this cycle, at the 11th hour they switched their focus from the council races to humanity as their favourite choice for reaper dreadnought-hood (one assumes the Asari or Turians had previously been the prime candidates.) So going pretty much straight for the largest concentration of humans in the galaxy makes perfect sense, since that's really all they're after.
Ah, yes. It's the "Humanity is special" trope completely made up just to put even more emphasis on Earth and humanity that still doesn't make any sense. If humanity is so uber special that Reapers can create a super incredible reaper (which can be destroyed by three people in it's early stages), why do humans make the worst Husks?
Also, if the Reapers want to use us for making more Reapers, why are they killing us by the millions instead of developing methods to incapacitate us like they did with the Collectors? Are you honestly implying that just because Mordin developed a counter measure against the seeker swarms that the Reapers decided to give up on the whole thing entirely instead of making more powerful seeker swarms? Why do the Reapers use dead corpses? Don't our bodies rot and decompose after we die? At least the collectors thought of that when they took us unconscious.
You can argue about tropes all you like, but in fairness, it was a human fleet that took down Sovereign and a human leading a mixed race crew that hounded Saren at every turn. This didn't come out of nowhere. Plus, it wasn't that humans were "special", it's that they're the most adaptable and suited to reaper needs in this cycle. Hell, we don't even know for a fact that the Protheans were the ones turned into a dreadnought at the end of the last cycle. For all we know they ended up as a destroyer or just what we saw of the collectors while one of their former subject races were singled out.
As for the processing: I don't think it's the bodies so much as the raw genetic bio-mass. The recently dead a probably just as viable a resource as the soon to be dead. As for their methods; they're in it to process BILLIONS. What's a few million killed or converted to husks here and there? Seeker swarms are probably OK for taking colonies and small settlements, but a planet of 11+ billion is probably way too much to handle all at once. They'd have to isolate large groups and swarm them one settlement at a time. Very inefficient.
Keep in mind that it's said that the once on Earth, reapers didn't spend the whole time walking about shooting at the ground. They quickly went about indoctrinating world leaders, rounding up as many civilians as possible into prison camps and started feeding them into the processor ships.
Also, as I said they plan on this lasting centuries. Javik was born long after the extermination of his people began. Doubtless had things proceeded as expected, there would be more humans born as time went on, though of course with decreasing returns.