[Gbro]

Adam,
I to am a firearms safety instructor and an avid muzzle loader hunter.
My condolences to you on this incident. I will pray for a huge recovery for you.
I know what you mean by others loading while barrel is still smoking. I confess that I to have done that. I have always believed the powder would flash immediately if a hot ember was still present. Your experience/accident is an eye opener to all of us.
I use B/P only but have experimented with powder pellets. There is a remarkable difference in burn rate for uncompressed B/P substitutes.
How this would relate to your accident, I don't know but everything has to be looked at.
If your recollection, and that of the others confirms your time element at anywhere near what you posted, there is just no way to establish a reasonable waiting period between loadings.
I am with armerandsafe in the static charge, however everything is not always equal in this kind of situation.
I do firmly believe this accident has an explainable cause and with the right minds at work you will get an answer. That being said, the only reasonable way would be in a legal action. Money talks!

Thank you for sharing your accident with us.

Greg