If you want to argue that Cerberus having manpower and fleets to rivial the combined forces of all the other citadel races is not a retcon then you need to explain ME2 - why does Timmy bother with an unknown risk like Shepard when he could literally have sent thousands of redshirts to go looking for the Collectors? And why, having spent an ridiculous sum on reviving Shepard did TIM not allocate any assets to keep Shepard alive? Maybe a fighter escort or some scouts for the Omega 4 relay? Or how about some heavy armour?
And how the hell did the Turians board these stations when it takes hours for the combined fleets of the galaxy to take out Cerberus' defending fleets?
Cerberus did not have thousands of thugs in ME2. TIM's henchmen in ME3 are mostly random people he turned into mindless slaves via the technology he obtained from the collector base. There is your reason he couldn't just send army to kill the collectors. And Cerberus is not able to rival all combined forces of the council races. If you paid attention you might have noticed, that most forces of the council races are kind of preoccopied with getting their asses kicked by the Reapers. If Cerberus was the only antagonist ist would pretty much be a short war, but as it is the Alliance ad allies are under heavy attack and don't have much resources left to deal with Cerberus.
That being said Cerberus having a huge fleet all of a sudden doesn't make much sense I'll have to admit. Their increased manpower is explained though.
And when do the combined fleets attack Cerberus? One Alliance fleet attacks Cronos station, not the combined fleets.