I always got the impression that if not from the shields droping when saren´s body was vaporized, the reaper would have won. Someone sent a distress saying that the reaper was way too strong for the fleet.
There´s no way for shepard to be prepared for sovereign. Very few people had seen sovereign till that moment, and no one seemed to know anything about its combat capabilities. Vigil only told us that it was not invincible (it needed the geth as back-up) but still formidable.
Also, the geth were deployed so they would keep the fleets occupied when sovereign attempted to bypass the citadel control.
You gotta be joking way you say that it´s impossible to know what will be required to bring down a ship. If you´re dealing with a known threat, a ship of familiar design, you know the kind of defenses and weapons it possesses. So you can send a sufficient number of ships to take care of it.
Joker himself contradicts you, since he says they can save the DA. In other words, the fifth fleet has more than enough firepower to wipe out the geth.
Also, the game had to somehow let shepard decide. For the same reason the council lets you choose your destination between 3 places, and then offer virmire as an extra place. They should had been be the ones deciding what is gonna be your next assignment.
We, as players, even on a blind gameplay, seem to know more than shepard. We actually see the enemy fleet invading the scene, making the initial attack and the citadel arms closing. Not only shepard had to think and act fast, he had close to zero info on the major fight taking place outside, but he did not know what level of power would be required to defeat the single reaper.