Is it possible to do any sorting on the playlist?
No. There is no sorting feature. Sessions are sorted in the order they are added to the playlist. We have made a suggestion for that to be a feature in the future.
Can you mark a session as "seen" rather than just deleting it?
Is there a way to mark items on playlist as already viewed?
There is no way to mark the sessions as having been viewed. Deleting them from your playlist after you've watched them or keeping track of those you've watched manually are the only options. Feedback has been given regarding this.