First of all; I don't think the Destiny Ascension scenario is
quite the same as Arrival. I for instance, can reason that by allowing the DA to be destroyed effectively accomplishes the Reaper's 'decapitating strike' strategy for them. Also; at that time, we learnt from Vigil that even Reapers are susceptible to amassed firepower from a fleet.
I also think it's safe to assume that other reinforcements from Turian Peacekeeping fleets etc, will attempt to hook up with elements of the 5th at some point at best, or at worst fall into blockade formation to prevent Geth reinforcements around the Mass Relay.
We know as Commander Shephard though that Sovereign is
dependant upon the actions of Saren (thus the whole story of ME1) thus any actions Shephard can do to buy time is therefore a battle of attrition that Sovereign is losing.
Even if the worst case scenario occurs and that Sovereign is victorious, then the cycle is half broken anyway due to Sovereign's failure to decapitate the leaders of the Council and causing a succession crisis in addition to general confusion.
Someone With Mass wrote...
Just like how Renegade Shepard is a delusional idiot when he thinks that giving the Collector base to a bunch of terrorists will benefit mankind?
It works both ways.
Not quite; see what I think you fail to grasp is after the Reaper war, everyone would be at a near technological parity, which means Cerberus/humanity wouldn't be any more of a threat in the future than what they are now.
As for Cerberus workign for the Reapers; I have my own theories as to why, but I truly doubt it would be Cerberus just becoming another implementation of Saren. Why do I doubt? Because I doubt they'd 'repeat themselves so soon.'