As far as adult snakes not being affected by thiaminase, I unfortunately had first hand experiance that this is not true. My first garter (a Manitoba T.s.parietalis) was fed a diet consisting of mostly goldfish and other thiaminase containing frozen fish (I think I used smelt). She lived for 9.5yrs.....but met her end very badly with convulsions and total lack of neuromuscular control. I agree that it does probably affect younger snakes fast, just simply because they are in an excellerated state of growth and developement...but adults are not immune to it and over time they can suffer the same devestating affects.
One thing that worries me, is that it almost seem like we're trying to make excuses to feed thiaminase containing fish, maybe due to the fact that they are so readily available, rather then putting a bit more effort in and making sure that are snakes are provided with the best possible diets. I know some snakes will accept little else then fish or even live fish, but I think we still have to make it an effort to provide them with species that possibly won't cause them any problems. Maybe it's just me, but I'd rather be safe then sorry and I wouldn't want to put another snake through the horror of what I know thiaminase poisoning can do.