Ok, here's an update. I took a coffee break and went back to it. I discovered that after pressing down hard on diagonally opposite corners, I was able to seat the valve cover back firmly. I tried to tighten the valve cover nuts. One of them kept turning no matter how much I tightened it. I thought it was good enough. I started putting back other parts/bolts together. When I got to putting the ignition coils back, I noticed that an ignition coil seal (which may have slipped off the coil earlier unnoticed) was wedged by the valve cover near a spark plug hole. Perhaps this may be the reason the valve cover was hard to put back on. I thought, oh no, I now have to remove the valve cover again to dislodge the ignition coil seal. I started unbolting valve cover nuts. They all came off easy enough, except for the one that I mentioned earlier went turning and turning during the tightening sequence. Now it keeps turning and turning while untightening but won't come off. Any ideas on how I should get it off?

 

I posted a follow-up but it doesn't show up here, so I'll repost. I was able to reseat the valve cover and bolted it back. Then I noticed there was ignition coil seal (which may have dropped off earlier unnoticed) wedged between the valve cover and spark plug tube. I tired to unbolt the valve cover. However, one bolt keeps turning and turning but won't come off. How can I get it off?

 

Sounds like you might have stripped the stud or nut tot the VC. With the others off, you might try putting some "upward" pressure to the VC while you try removing the nut and see if that will help it some off....nothing extream as you don't want to bend/deform the cover causing "sealing issues".

 

I couldn't get anywhere further with the obstinate nut. I finally had the car towed to the dealer. They had to break the nut. They ended up replacing spark plug seals and one ignition coil with a charge for 2 hours of labor. The engine runs smooth now. I don't think valve adjustment was required otherwise they might have said something.