N. Needleman wrote:Commentary from a fan who met Sabrina Sutherland at the TP Fest and learned this:
David and Mark obviously co-wrote the series... to a point. Around the time they handed things over to Showtime, things changed a bit. Mark went off to write The Secret History of Twin Peaks and David directed the whole thing, but the interesting thing to note here is that, as you all may recall, the series Showtime announced was going to be 8 or 9 episodes. There was that whole drama where David announced he quit, Showtime caved, and now we have 18 episodes. This was NOT a case of the existing material simply being stretched out. According to Sabrina, David actually continued to write. So this seems to have shifted the level of writing involvement more heavily to Lynch than Frost. EDIT: For clarity, it's not that David wrote nine more episodes. It's more like he wrote more scenes interspersed throughout the existing material.
More here, including Jan D'Arcy talking about how Eamon Farren was so upset by the assault scene in Part 10 he called his own grandmother.
I think this closely matched what Fenn said on a podcast interview that was quickly pulled down. Since that interview, I suspected Lynch wrote a significant amount of the new material himself, if not all of it.
Which does not, in my mind, in any way, shape or form diminish Frost's contribution. You can still see Frost's fingerprints all over S3 (Dr. Amp anyone?)