Thanks for all the suggestions about how to proceed.

The torch I used was a small butane one that delivered a pretty focused flame to the screw. Perhaps I should have left it on longer, but I was hoping to be able to free the set screw without discoloring the metal.

At this point, I believe that I will be able to remove the canvas from the frame without having to mess with that setscrew and its fitting. So I will not be messing with it more unless I have to.

I do think that the most reasonable next step will be to use an angle grinder to carefully cut the fitting off the tube...IF I have to go that route, but I don't think I will. I would assume that as hard as the screw extractor is, it would be very difficult to drill it out. Carefully grinding off and replacing the fitting would probably be easier and pretty inexpensive. But as I said, I hope to be able to just leave it where it is and ignore it until I really have nothing else to fix on the boat...;>)

Thanks again for the suggestions!

As a rather new boater, I really appreciate being able to ask for advice from this group!

Thanks,
Nick