One thing I hate about discussing the trilogy in general

C-Sec doesn't contain the sum total of all C-Sec officers on the Citadel. There are multiple precinct around the Citadel. It is needed because the sheer size of the space station.  Since you know it is roughly 24 miles long and has multiple arms each equal length.  Might be a main or the main C-Sec office but doesn't mean it will be full to the brim with ever C-Sec officer.

He is using his Geth army they are in a space battle keeping the hundreds to thousands of military troops with military grade equipment busy in space.  Geth have the capability to land troops on their own even drop them out of low orbit right to the ground.  Not to mention as I think was UpUPAway or maybe Hans said regular ships docked with fake credentials or searches waved off by Saren's authority as Specter to store Geth in to attack once the Space battle started.   The Master Control Unit that Saren wanted/needed is located in the Citadel Tower which isn't C-Sec's location.  The attack on C-Sec only existed to wipe out the air traffic control/troop organizer. You don't need a massive army to raid the place, plant a few explosives and blow the place to hell. Thus preventing C-Sec from being able to organize a resistance.

The plum and only reason for ground assault would be the Citadel Tower and more specifically the Council Chamber.  And we already have Ashley's soldier examination of the layout to go by how well you could defend from in it from an outside force.  All for the literal minutes it would take for Sovereign to cross the space between the Relay and the Citadel Tower.  Once Sovereign docked it doesn't matter if the Tower gets taken by C-Sec forces. Saren might die but Sovereign doesn't really give a flying fuck about the life of Saren or any other indoctrinated person.  And without the super magical inexplicable program that was some how created by the Prothean Scientists and sent to the VI on Ilos even though their communications were destroyed as the VI already said. Nothing short of another super advanced AI like EDI would have a chance of delaying Sovereign accessing all the systems it needs to.  Meaning it is game over for the galaxy. Reapers are called in and harvest is started.

Saren messed up on so many levels.  These are just the more obvious ones. As UpUPAway said Sovereign knows how the Citadel works. It would have been hilariously easy just to have Saren examine the hidden mechanism to understand why the Keepers were not responding. Then capture and indoctrinate some Salarian or Asari Scientists and have them fix the problem. Boom Sovereign can now remote trigger the Citadel and game over.  The Conduit is completely and utterly irrelevant to the whole attempt and has absolutely nothing to do with the plan to calling the Reapers. The only reason the Conduit has any point in the game is because the writing forces it into the lime light by having Saren be so unbelievably stupid and making Sovereign so unbelievably stupid to force it into being meaningful in anyway shape or form.

I have no idea why you are trying to defend the undefendable. I have a few theories. But please keep trying.