Like any other hardcore fan, I'm quite sure Brandon could have managed it with the proper time and effort to do it.

It's much like his bad habit of replacing minor existing players by picket fences with fan names. It did save him a whole lot of time which, considering the pressure/deadline Harriet imposed, he probably didn't have.

A whole lot of the problems came from that. Brandon first drafted two story lines and a half thinking he would finish the whole thing and then step back and look at the whole before rewriting parts that didn't work so well, incorporating stuff he had completely missed etc., which is normally his own work method.

Instead he underestimated how much work remained, he ran late and, long story short, they ended up putting together a book out of his two story lines, with the rest still existing only in vague outline form. A whole lot of the efforts went to structure TGS, write the missing scenes from other story lines etc.

The time when Brandon should have stepped back to look closely at the OP mechanics he used by instinct while drafting (which is okay ... he doesn't want to kill his momentum and even with his own magic systems he makes mistakes or turn corners short in that phase of the process), validating or changing them by comparing what he did to the notes and past books never seemed to have happened in the end.

The two other books were similarly rushed, in the sense that there was so much stuff to revise in editing to make it work and avoid continuity errors etc. that it seems there was little to no time left to polish, beside the fact that a lot of things were not optimal because they had been published before Brandon got to the end of the drafting phase (resulting in stuff like the envoys of Egwene spending way, way too long a time at the BT to be credible etc. That one came from syncing Rand and Egwene, when RJ pretty much had set things up for most of the Arad Doman story line to take place before Egwene's first scene in AMOL. In his restructuring, Brandon decided to have an ellipse of a few weeks for Rand before his first AD scene, to catch up with Egwene, who was given a long story line in turn (He basically undid what RJ had carefully aligned in KOD, by having a whole month of Egwene's story in one chapter, and by jumping forward with an Elayne wrap up and Mat having left Tuon... in order not to return to Egwene or Mat at the beginning of AMOL... focusing on wrapping up Perrin and starting Rand's descent into madness for a while, before gradually re introducing Elayne, Mat and Egwene when the chronology caught up with them, and in the middle of the Rand mess and Perrin being stuck. RJ's outline planned for Egwene to end up jailed almost right away. Brandon decided he'd rather have two dinners with Elaida with RJ's planned outcome happening in the second. This way he had an Egwene story line to play out against Rand's. But it's no wonder he ended up needing three books to tell what RJ had set up to happen over one very long book.).

No doubt the book would have been better had Brandon spent the whole time it took for the three books to come out finishing the whole thing first, than revise and optimize it (as RJ himself would have done), spend all the time necessary in editing and as many drafts as necessary, before splitting the whole in two books for publishing purposes. Had Brandon done that, perhaps it would have become obvious why RJ had made some decisions (like the much shorter pre-TG arcs for Egwene and Mat) and his intent for the structure of the story would have become more evident.

But then, there was the impatient part of the fanbase already unhappy not to get the whole thing by the time TGS was released, so they had pressure and hard decisions to make. Quality has suffered though, there's little doubt about that.