That's not quite correct though. There's always a "left-behind" Reaper after a cycle. It was Sovereign during the last cycle, he never 'returned' in dark space with the rest, and was never really "sent" from there back to the Milky Way. He remained in the Milky Way (safe to assume that the same happened in all previous cycles, since other cycles were never broken, the formula probably never changed) and periodically assessed the evolution of organic life. There IS, however, the "On switch" (figuratively speaking). Which I guess can be considered cringe-worthy (story-telling wise).
The problem is, if it wouldn't have been necessary for a Reaper to 'activate' the Citadel manually (such as what Sovereign attempted to do in ME1) then there would have been essentially no story to tell, and the credits would have rolled right after the intro in ME1. The Reaper(s) (be it Sovereign, or any others in dark space) would have activated it automatically from any 'x' distance and there you go, everyone is dead, no trilogy worth doing out of that.
So really, there's no need to hug your Normandy plushie (well I admit, if I had a Normandy plushie I would hug it regardless). Rest assured that ME1's story, overall, remains the best in the trilogy.
Wasn't there the whole thing about the keepers being manipulated to prevent their action in that regard? They were the ones who were supposed to take care of that. If it's a giant mass relay which accepts external signals, those signals can be potentially copied, and the whole process fails. I interpreted it as a measure of using a less internally secure method to obfuscate the implementation rather than a more secure external method which potentially exposes the process to more attacks.
Then again, starchild throws that all out the window.