ThoseDeafMutes said:
Ships are undetectable while at FTL because of light-speed lag (explicitly stated in codex), so they could also simply FTL up to the "top" of the galaxy and manually FTL to the citadel from there, with nobody to interrupt them until they arrive. This would put them in the same situation Sovereign was during ME1, attacking the citadel fleets, only this time there's like 200 reapers instead of 1. They can manually take control like Sovvy was going to and do whatever.
You have to do some pretty impressive mental gymnastics to make this work assuming they can fly bacck whenever they want. I suppose you could say "they were asleep and couldn't wake up until their alarms went off", but then they still should have come at the start of ME2 instead of messing around with the confusiong collector plot (the final goal of which is pretty ambiguous, since another single reaper doesn't help them get back to the galaxy at all, especially when the races are all rocking reverse engineered reaper tech now).
They're not
undetectable - their signal just doesn't travel faster than the speed of light, so by the time you see it, they've taken off. There would still be a 'wake' left behind, and any station that happened to be inside of that before they got to the relay would be able to phone ahead and go "Holy
shit guys this radiation, something's coming, might wanna lock the door". They'd still be able to spread around pretty damn quick, but there would be enough time for the Citadel to scramble its defense.
And while 'tons and tons of Reapers' is still probably a losing battle, it might not be so entirely one-sided; remember, Sovereign fucked shit up, but it was Sovereign
and a gigantic Geth fleet that did the attack. It's very doubtful that an individual Reaper can fight a proper fleet to a standstill (if they could, why bother with the Rachni and the Geth in the first place?). The Reapers would probably win, but it's quite likely they'd take some pretty heavy losses, which I'm sure they wouldn't be too happy about.
Same with flying back "whenever they want". While yes, they technically
could have done that at any time, there's no telling what kind of toll it would actually take on them. The energy expenditure alone might be massive enough to leave them 'starved', weakened and vulnerable. Combine that with losing their element of surprise and being unable to yank communications out from under the galaxy's feet, and not having instant access to all the central records, plans, and information they could otherwise datamine from the Citadel, and it's not too far-fetched that this would be their absolute last resort.
Given the choice between figuring out a way to restore their original, safe plan that all but guarantees victory, and a method that will almost certainly result in taking some substantial losses in a war (and could quite possibly result in them losing and dying, as we all know it will), hanging around in dark space until their other options are exhausted makes perfect sense.
Edit: I honestly really don't know how exactly the human reaper in ME2 was supposed to help things, other than maybe giving them another 'Sovereign', but besides that, if this is the way the basic plot is going to go, it actually makes a good amount of sense and lets the story come to a close without either declawing the Reapers as a credible threat, or requiring some eleventh-hour deus ex machina solution to the problem. (There still may well end up being one of those, but who knows?)
