I had two stubborn edges that I wasn't able to clear as seams and I tried everything I could think of (selecting vertices -> Edge -> Clear Seam, Selecting edges and clearing, Re-marking seam and trying to clear, etc.) and the only way I was able to eventually clear them was to SELECT ALL edges and the Clear Seam which removed all of them. Luckily, there weren't many edges to clear in my wooden board and redo and I also had some incremental saves that I could roll back if needed. Are there certain situation where it won't let you clear a seam or was this maybe just a weird bug (I'm using v7.80 RC3)? I feel like I'm missing something crucial. Kind of like how edge loops won't select triangles - is there some rule about seams or is this a quirk/bug?
I have caught myself adding sharps instead of seams during this exercise a few times, so that may be it. Another newbie thing I learned (in case it helps someone else) is I thought it wasn't letting me add seams/sharps, but it was adding them and just I had the view turned off in the Overlay settings. It turns out the Overlay settings are unique to each workspace. So for example, if you enable or disable the overlay View Seams setting in the Layout or Modeling workspace, and move to the UV Editing workspace the settings apparently aren't shared between the workspaces. It took me an embarrassingly long time to figure that out! This Treasure Chest tutorial is great and I've already learned so much from it - thank you Kent & CG Cookie for sharing your talent with us and for all that you do!!!