Every theory that relies on several things happening just so becomes increasingly more unlikely the more factors are added.
Even without that, there is one huge problem in the theory: Every species was giving their all for the final battle, all their resources went into the Crucible or their fleets. Who held back and didn't commit fully to the war effort so they could build a second fleet? Hackett? That goes completely against the character.
I mean, building the Crucible was insane already. "Hey, we found incomplete plans of something we don't know what it is and we want to redirect all resources to build it. No, we don't know what it does and it seems no cycle before us got it to work." - "Sure, let's build it, it might be our only chance!" *headdesk*
And now you're proposing that a second insane project existed and people built it, too? It was unbelievable the first time... I don't even have the words to describe the second time.
Of course, BioWare is probably going to make it work as long as you don't look at it directly. Because no matter how you spin it, withholding the resources and personnel means backstabbing Shepard's efforts to defeat the Reapers. The entire galaxy comes together and throws everything but the kitchen sink at the Reapers and some shady backroom dealers decide they would rather run away? And those are the "heroes" of the new game? Well, let's go all the way and play as villains then. Let's conquer Andromeda! Nothing we do here will be half as bad as condemning an entire galaxy to their fate for selfish reasons.