That's why I brought it up, was to possibly keep someone else from going through the same thing you did.
( we never used smoke machines, but more a solid set of diagnostics reinforced with experience) :D
Precisely why I posted.
I asked around the site for months and nobody ever offered up the throttle shaft as a possible leak... or really offered any detailed explanation of what would cause the idle to change. I'd say using smoke to pinpoint a leak IS solid diagnostics...and experience comes from trial and a lot of being wrong. ;-)
Hopefully the OP has an easier time finding the problem.