What I found about benfotiamine includes:

In vitro studies (test tube work not using living systems)
in vitro studies may or may not translate to living humans.

Animal studies. These may also not translate to humans.

I did find some comments about not using lipoic acid if you are thiamine deficient.
I do not know where those comments come from.

Benfotiamine comes to the US from mostly German use.

I found a mention about befotiamine being converted to thiamine when passing the GI tract. Thiamine itself does not cross GI membranes well. So some of the benfotiamine successful use may pertain to just MORE of it getting into the blood stream this way. We have some discussions on our PD forum ...they are using high dose thiamine (and some are using benfotiamine) to help their movement disorder. You can search those posts on the PD forum using "benfotiamine" and "thiamine".

Rashes can be from many things. If a rash occurs after you use or eat something, stop using it and see if the rash resolves. Then reintroduce the suspected item and see if you get the same rash back. Don't confuse a rash with just flushing or pinkness. That may be a vasospastic event and not a true rash.

It sounds to me like you are conflicted about B1. If I were you I'd just forget about it. If you are of Asian descent you may have a genetic error in thiamine metabolism. These individuals cannot metabolize some drugs well, or alcohol.