I happen to have a scope-cam and took a short video of what I found today while soaking the valves with PB Blaster. Here's a link to that video -
https://photos.app.goo.gl/Sro1BG9P1i9JLNbM9
The sticking valve is the #7 INTAKE valve. When I crank the motor the valve lifts as it should but does not return. With just a little pressure I can push it back down into place. I then hand-turned the motor till the valve was just a little open, and using a 90 degree awl I was able to lift and lower the valve with very little resistance. I can also spin the valve fairly easily.
So I'm assuming I have a broken valve stem or spring. Sound right?