He "left abundant" notes about the Sharans, and about Demandred through the series, apparently. Brandon tried to work some of that into the book as a sequence of scenes that, from his description, sound very much like some sort of flashbacks/reminescences. They thought it hurt the pacing/flow and seemed out of place in the middle of the action, and removed them.

No doubt there will be non-cannon information in all of this, but Brandon has hinted it will also feature unexpected information (some were asking him about old stuff, the purpose of the EOTW etc. and he said "well, River of Souls" will answer some of those questions). And more stuff in the vein of Demandred recruited Taim.

Unless Brandon created this san'angreal, which is always possible, I think he simply used RJ's notes about it/Demandred/Shara.

The Encyclopedia is more likely than the novella to provide us with a whole lot of new information on Shara from RJ's notes. Most of our information come from the BWB, which already had a surprising amount considering how little had been revealed at that point in the series.

It's not impossible Brandon invented Sakarnen, making it the san'angreal mentioned by Lanfear etc. He likes his gadgets. The whole story about Rand getting Hawkwing's Justice and giving it to Tam was wholly invented by Brandon, for example.