see shooting script of last episode (written by Frost) vs filmed final episode (directed by Lynch).
I'm not sure Frost wrote much of that script, but we'll probably never know. It sounds a lot like Peyton to me, with some of Engels's ideas thrown in. In fact, Frost's non-involvement (he was basically absent for the whole shooting period of episodes 17-23) was in all likelihood a much bigger problem than Lynch's, since Lynch disappeared for a lot of the first season without any noticeable drop in quality.
I think Frost outlined the main story of the last episode, which Lynch mostly stayed true to. There are actually few major changes except to individual lines, and then mostly in the Red Room. It's the over-the-top Windom-ness that really feels too much.
Peyton is a seriously amazing writer, but he needed a good editor. With proper guidance from Frost (and Lynch), he wrote Laura's funeral (with Shelly's joke) (1x03) , Albert and Coop's breakfast (2x02) and "Can you see me, Daddy?" (with Engels; 2x06). Without guidance he wrote the Evelyn Marsh plot.