Both the forest and the late added character were there to give opportunities to shake things up/lighten the mood. This was a grim, grim season and you've gotta have some of that in an 11-hour runtime.
I agree on Saw's part; felt like something they had to get through to complete the series' connection to Rogue One. I did love the season other than that, though. "Who Are You" is one of the best episodes of TV I've ever seen.