He did read the series many times, his last re read is even recent. Those he made not as fan but as writer he made focussing on very specific aspects, among them spotting all the stuff he found relevant to the finale.

What you suggest about the series isn't reasonable. It's the outline he has to work from he has to know at that level, not the full series. Your standards for Brandon are way, way too high. He's fleshing out the outline for the last book, not writing an encyclopedia. He's not supposed to have developped encyclopedic knowledge of all the books, he's expected to have spotted in them all that's relevant to the part of the series he has to write and that isn't listed in RJ's notes.

His mistakes during Q&A don't bother me. They annoy me a bit because RJ's astounding memory of his stuff really spoiled us for years (few writers can even do Q&A like this, and few writers include minutia at that obsessive level in their books because without an exceptional memory like RJ's they'd have their nose in their notes every two line they write, or would end up with inconsistencies in their finale novels. RJ didn't because he had an exceptional memory, and it seems that details he didn't remember he knew he didn't and looked up. That didn't stop the hordes to jump at him for the rare mistakes that slipped in the books... how dare he mix up an AS' Ajah once and what not...).

People have too high expectations for Brandon and ask him questions they should keep for Maria Simmons. When I ask him at a pre-TGS Q&A about Jordan's "Power ranking list", Sanderson admitted he barely had to refer to it to write TGS, and not at all the list for men. The most time he spent with it was to satisfy his fan curiosity and at the time I met him this had taken place two years earlier. He remembered Nynaeve stood at the top, but at the time he couldn't even remember if the list included novices or the Forsaken women... It's not likely he had to use it much for ToM either, the short version Maria told him RJ used most as he wrote listed strengths of each AS another character interacted with most often, and so must have been essentially a reference for levels of deference of minor characters when he wrote AS scenes. The longer version of the file had details like which weaves/talents RJ had established in the books as something a particular AS knew or didn't know.

He had, for e.g., totally wrong assumptions about Cyndane's strength which Maria had to correct after a Q&A blunder about that and it incensed fans like you who are interested about such thing. But the reason is all too simple and obvious: not a single Cyndane scene or mention has required him to refresh his memory on details like this. It's exactly the same for Moiraine now. All the ToG/Moiraine scenes in ToM were written by RJ.

My knowledge of certain areas of the series doesn't come from re reading the books over and over so much as it came from discussing my favorite aspects over and over for many years on MBs. I did many more than ten readings and tons of details escape me, and when I want to discuss them I have to go find resources to verify the information first. And for me that includes the OP details that I never pay much attention to. There are many topics in which I could answer very nitpicky quizzs and get nothing wrong, and others the next fan with other interests would consider "basic stuff" that I would get wrong.

You've committed tons of OP details to memory that the average re readers haven't. Give me a list of AS, and most likely I won't be able to order them by strength. I remember the basics, like Siuan/Moiraine/Elaida being at the top, and Daigan being very low. Don't ask me about Myrelle, Verin and co. Whenever their strength was relevant, RJ reminded me in the text and that was always enough for me. Can you keep track of all the Sitters in both Hall and their scenes through the series? That I can, because it was of greater interest to me and I've discussed them over and over.

Also take into consideration that in Q&A Brandon doesn't have even 2 min. to think before he answers.

I'm far more concerned with all the little mistakes that have escaped all the editing passes and none too pleased that for the last two books they forced themselves to keep deadlines that forced them to rush these things. But note there is no discrepancy in TOM about Moiraine's strength or her angreal. There is only one in this Q&A because Luckers asked Brandon a question about something he didn't remember for sure but thought he did, so he answered. When he knows his knowledge is too incertain, he refers to Maria (his MAFO)

But I don't expect Brandon to develop encyclopedic knowledge, certainly not in the most detail intensive areas like OP, minor characters etc. He has RJ's assistants for this, and most of this stuff won't even be in the least bit relevant to what he has to write for TGS/TOM/AMOL - encyclopedic knowledge of those details would only be to indulge fans during Q&A. Pointless. When it is relevant, he's very much aware how obsessive the fans of the OP can be, and he no doubts make sure to check in the notes, or Maria does (Brandon can't really let minutia interrupt the flow of his writing... you look up the notes in preparation, write as it comes and later do facts checking and corrections).

Despite the assistants's help with the notes (which I'll remind you Brandon said a few times they are longer than the whole series...) he's made several "big mistakes" nonetheless (eg: his first scene in Malden in TGS, a rushed last minute addition after the decision to split the book but include Mat and Perrin scenes that were not yet written and that were like "prequels" not planned by RJ flagrantly contradicted the last scene in KOD where he left Malden first and the Seanchan army stayed behind and they missed that), but on the minutia like OP details and respective strengths they've not even really blundered in a major fashion so far.