Mike, having deposits on the back (stem) side of the valve and smoke on deceleration (high intake vacuum condition) has just GOT to be a leaky intake manifold, loose valve guides or bad guide seals...you just can't get
oil or deposits there any other way that I can think of (except for a PCV sucking
oil, and you would have an oily PCV hose and/or valve in that case). And you've pretty much eliminated the leaky gasket. A remote possiblity would be a leak/crack in the intake manifold that would allow it to suck
oil, or maybe pushrod holes in the head?
I would suggest changing the seals first (fairly easy), and if that doesn't fix it, welllll...you know what's next. I don't believe it is your rings.