@Fairygodmother. Hi, I'm just a layman, but have been doing a lot of reading around the diabetic pathways, and looking at what supplementation could possibly do to minimise the damage each one does. I started taking Benfotiamine around 10 years ago when I first heard about Thornalley's work with Thiamine, but switched to Thiamine when the clinical trial results came through.
As for Thiamine, Thornalley and others discovered that Diabetics are deficient. The mechanism seems to be increased renal clearance (30x normal). This could be because kidneys under higher-than-normal sugar loads are less able to reclaim thiamine before it's expelled in the urine. There's just something about the diabetic state that causes this I think, but I'm not sure anyone has answered it yet.
My thinking on this would be no Thiamine = no Pentose-Phosphate pathway = more sugar passing through the bottom 3 harmful pathways. How much of a difference would it actually make? We don't know. The clinical trials for Thiamine have been concentrated on the kidneys, where it seems to have a large positive benefit.
Side effects of Thiamine? None that I have noticed (and I take a massive dose). But if anyone knows of studies that show adverse effects of Thiamine, I'm happy to learn about it!