Hey Suzoo.
Allways nice to be put to work!
Concerning your past feeding with thiaminase-consisting species, you don't need to worry.
The Thiaminase is a highly selective enzyme, created for one single purpose, which is to break down Thiamin (Vitamin B1). (quote from my dear father, the biologist)
That means that if your snake hasn't shown any signs of lack of vitamine B1, it will probably have no further problems, that of course if the diet is restructured, and the new diet is sufficient.
By the way, you can feed your garters with prey, that contains thiaminase as long as you give them a B1 supplement. You should ask an expert for the exact amount of supplement, as too big amounts of B1 can be harmfull to the snake as well.
I find it hard to believe that a home-testing kit for thiaminase is be found on the market, but I have been surprised before...
try making contact with some zoological researchcentre near you. maybe they can come up with some more results, than the ones in the book.
Concerning the worms, I'm afraid, the only thing i can tell you is that red worms are the tiny little buckers found in your composte. It is not the same species that you typically find in your lawn. maybe you'll come across a few, but you can allways look up some pictures on the net to compare.
I am not sure that I have the same "wormfauna" here as you do in the US, so an american would probably be able to help you better than me. (With the lengual barrier in mind)
hope this is will be to some help for you.
Tore