I'd say the reverse tolerance thing is still unproven - although I think a lot of people swear by it. It could just as equally be the ability to hold the smoke, or burn it the right way - to get the Salvinorin vaporised. I haven't seen any scientific work done on the subject so I'm staying sceptical about the whole thing.
I may be talking out my ass (just throwing ideas around), but I wonder if more receptors are made - and because there isn't a tolerance, it just enhances the effect, I think I read something about increase of receptors (adenosine?) that bind with caffiene, but it could just be me imagining (*Shrug*).
As for permanance I couldn't say. Sorry. Because there hasn't been a massive amount of research on salvinorin you may just have to do with refering to the behavior of other chemicals which have had more focus (dispite the obvious differences).