My vote goes for Valve Seal also. In my experience, when oil control rings go, they smoke alot all the time during driving. And normally, if they are going, you got major things going on. But you can't rule it out. Oil rings can sometimes get stuck in the piston landing. It is also possible that the #7 cyl got washed by fuel and rings are not sealing anymore. The valve seals will drip oil down while sitting, which is why it smokes on start up. Compression test may not be conclusive. Compression rings don't always go at the same time. Sorry to hear about this, not an easy thing to figure out unless the motor gets torn into.