You seem to be missing the major piece of the puzzle here.
Sovereign is in contact with the Reapers whenever he is awake - with the Harbinger. It doesn't need keepers to communicate the problem to them.
When sovereign fell - Harbinger used Collectors to evaluate the situation. It communicated from the Dark Void with them.
The only purpose of Keepers is to keep the Citadel in prime condition so it will attract new species and when the signal from Sovereign comes - activate the Citadel Relay.
Reapers don't need Citadel to come. It's just a due process - the most effective one. Sovereign failed to activate Citadel - Reapers still came.
They come every 50k years either thru Citadel which was successfully activated or thru Alpha Relay or as in the last cycle - just fly to the galaxy.
They knew everything that happened to keepers and they didn't need to do a thing about it. The plan was - for Soveregin to activate Citadel when the time comes by any means necessary and if that fails - Reapers would be able to go thru Alpha Relay and as the last resort they can just fly.
They are unstoppable, and they knew that. The last cycle practically lost if not for Crucible - the only mistake Reapers made.