Snapshots are stored copies of the file as it was saved when you took them. Just like loading an RTF file off of the disk in Nisus Writer Pro or Word or something, there will be no undo data left. Undo is only saved in the session while you are working and is discarded when closing.
Yes you can drag a snapshot from the list into the header bar for either editor split to load it there. When holding down the Option key while dragging, you’ll get a comparison view.