Brandon don't say anything about "flashback scenes". He used the word flashback inaccurately. To paraphrase him he said:

The book doesn't only or mostly return to the timeline of TGS the way COT did for WH ("it's not 100% flashback like book 10". It also moves the timeline forward significantly, and so we pick up with Egwene and Rand along the way, but first with Perrin (and Mat, but Mat is less behind) and many other characters we will return to the timeline of TGS so everything can get in synch before the last act of the book.

It's nothing new, and no mystery. In TOM Rand and Egwene will first get a few bridge chapters that lead up to the part of the book where a big event happens that reunite the main players and is the beginning of TG. We've known about that for well over a year now. Typically RJ's third acts cover the last third of the book, so yeah, Rand and Egwene will have a significant role, first about as much as Perrin/Mat in TGS for the bridge (so Brandon explained), then their involvement in the last act.

There's absolutely nothing that increases or decreases the likelyhood of a Graendal scene in there. As before it still depends if she is or not involved in a minor storyline that will return to the timeline of TGS in TOM. If she isn't involved with Aran'gar or the other Chosen, it's unlikely she will appear (I can't see her getting involved with Mat/Perrin when she had to be in AD to scheme against Rand). If she is still involved with Aran'gar, and the TGS prologue suggests a further development may be coming after Aran'gar has lost her position with the rebels, which she not had yet in the TGS prologue, Graendal may very well appear (those "flashback" scenes Brandon preferred to cut from the timeline of TGS could all have been moved to the prologue of TOM, incidentally: Graendal/Aran'gar, Alviarin stuff, Red envoys, etc.). Etzel read this as "it's 100% flashback-free" where Brandon said "((TOM is) not 100% flashback". Brandon said nothing about flashbacks, in the proper sense.

And he also all but confirmed that characters we haven't seen in TGS like Aran'gar, or Alviarin will appear in TOM.

Somewhere else he distorted what Brandon said about Graendal's death too by twisting the context of the question. He refused to confirm her death before TOM is out (where he said the answer might also be obvious), because dead or not, her role is not over. All he was willing to say was that she will at least be mentionned again (which is not quite the same as saying "she will only be mentionned". He opened the door to a possible reappearance, and didn't want to hint if it happened before her death or meant she isn't dead, was on screen or through a mention by someone else. That's all.

Etzel should ask himself why Brandon, who is a lot more forthcoming than RJ and doesn't like to mislead people on purpose (he said that) unless he is forced to because their questions involve spoilers, didn't just flat out say since, according to him, there's no mystery that Graendal's role is now 100% done for in the series now : "Graendal is dead. She won't have any more scenes". Obvioulsy, there's something ackward for Brandon that prevents him from confirming flat out we're done with Graendal.

IMHO, there's a fair possibility the ackwardness arose from the fact Graendal is dead by the end of TGS, but she won't be dead yet for the first half of TOM, and he didn't want to hint if she will make a return before she dies or not. He went with a non-committing "she will be mentionned again", which leaves all doors open: she may not be dead at all, she might make an onscreen reappearance pre-death, she might be involved in the pre-death timeline because she will be mentionned by other characters (Aran'gar, Demandred/Mesaana etc.) or TOM book may deal with the aftermath of her death (Forsaken meeting etc.). All of this is possible (though personally I can't see her having survived Rand's attack).