So, after trying everyone's suggestions, scouring the internet, re-reading the manual multiple times, I finally called Xfinity (Comcast) again - fully aware that a technician who determined that the problem was user error would be costly. My time is worth something, too, though, and I've spent WAY too much of it on this TV! I thought the hurricane would have led to a very long wait for a technician, but no... I called yesterday and he came today! He spent about five minutes checking connections, etc. and then swapped out the box for one in his truck. A few minutes later, after reboot, it worked perfectly. There was nothing wrong with the box per se; it worked fine with the other TVs in the house, and it's a box supposedly designed for 4K, but it didn't work with this particular TV. The tech says they see it all the time. The new box is a slightly different model, but it otherwise the same - and it works exactly the way it should. THANK YOU to everyone that pitched in to help solve the problem.
