Collectors aren't useless. They were created as disposable front line troops much in the same way husks or marauders were. After their purpose was for filled they altered them again to the current state in game. They lack the numbers and ability to replace said numbers if killed to be an effective fighting force in a prolonged battle.
4. Why call the Reapers in the old way? Wait it out till the time and settings are perfect and strike. Open the relay and send the signal to the Reapers to flood in. You act as if the Reapers are desperate to attack. They really aren't.
5. So numbers we don't actually have. Estimates people think exist, ignoring any mixed species planets/locations and that is how you come to your conclusion? Seems like you are missing some data that is needed. I do not doubt that Earth contains the largest population of humans. To many unknown variables to make any specific claim. Particularly when you are out right ignore mixed species area.
6.Most likely Legion meant that time and time again Sovergin would sit and wait for the correct time to activate the signal and call forth the Reapers. Needing to act though outside agents so as to keep it's existence a secret. Most likely there were times they were needed earlier and times needed later. Other wise they could have just programmed the citadel to activate automatically every 50,000 years.
Every relay in game shows to be used you have to approach the relay before using it. Maybe the Citadel is special and can avoid that one requirement. Who knows. But as for why would they wait. Why not? I can not think of any good reasons to not delay if it gives them a chance to speed up the harvest. The 50,000 year cycle isn't tight schedule that has to be done by then or else the entire plan falls apart.
7, If Sovereign's plan worked the galaxy would have know about the Reaper's anyways when they started attacking.
8. Vigil was a VI stationed on a planet that went dark almost as soon as the Reaper invasion started. Vendetta was also a VI implanted on Thessia and again abandoned shortly after the Reaper invasion started. All relays connect to the Citadel. It is how they ensure each new cycle will pick it as their place of authority. All roads lead to Rome type deal.
9. It can communicate with Reapers. The question is why would it want to interfere when it continues to study each cycle? The AI isn't malicious, its set that the Reapers are the only option for all eternity. Considering first chance that shows up when it sees the solution is failing it offers you a chance to change it.