[noggin]

A further observation is that when the Smart YouTube TV app plays 1080p content, TV still shows 4k resolution.  It seems to switch to appropriate frame rate unless the original rate is 30Hz, in which case the TV displays 60Hz.  Is that because I used the 4K option first time I loaded it?  The 1080p content doesn't look good via that app.

30Hz stuff output at 60Hz and 25Hz stuff output at 50Hz is not a major issue. I wouldn't worry unduly about that.  All that should mean is that each frame is output twice via HDMI rather than once.