loremazd, pretend it was a best case scenario.
[ul][li]The Protheans have not sabotaged the Keepers.[/li]
[li]Everything goes as planned.[/li]
[li]From the edge of the galaxy, Sovereign tells Citadel to "do it."[/li]
[li]Citadel activates and the Council races are boned by the sudden Reaper fleet.[/li][/ul]
No, Sovereign didn't have to be there. But he had developed a bit of an ego and wanted to flaunt it off. Chalk this up as another plot hole. He didn't need to be there.
Yes, Saren could have done it on his own, just as surely as the Keepers HAVE BEEN doing it on their own, for however many cycles the Reapers have been using the Keepers to do it.
And yes, Saren could have activated the Citadel without alerting C-Sec or whoever. The Keepers have been able to pull it off without arousing suspicion - Saren is a veteran spy.
And you still haven't answered how either Saren or Sovereign knew about the Conduit in the first place. Before Eden Prime, Saren knew about the Conduit. How? If Sovereign knew about it, then why'd the Reapers let those Protheans escape? Over the course of the 50,000 years of waiting, why didn't Sovereign find the Mu Relay himself?
Liara said it'd take decades to find the relay by conventional means. Sovereign has that time. Instead of hibernating, why didn't Sovereign repair the Keepers?
The Reapers left pieces of their tech scattered about to guide the galaxy in technological development. What if the Krogan had found Citadel station first, instead of the Asari? The Krogan would have fucked it up and in doing so, fucked over the Reapers. What if humanity had found it first? We'd have dismantled it to study the technology - that would have sabotaged the Reaper's plan.
And yes, I can go on. The plot holes abound.