Well, a bit of an update. I took my own sweet time trying to get ahold of the the shop owner. I did so today. He seems to think it's a bad hydro-adjuster pumping itself up and locking the valve open. This seems plausible...

His explanation for not thinking it's a valve/seat/guide is that in his experience, a valve that sticks stays that way. The head is out of warranty now, so I can't see him having much desire to lie to me about it. In fact I'd think he'd want me to bring it in and spend money.

As for the high idle issue, I found it and fixed it...but I'll be damned if I remember what it was! (was something small and stupid)