This sounds like a very good assessment of the situation, specifically regarding the amino groups. Not something I know much about, but definitely, the DT mechanism relies on a special relationship between certain types of dyes & gelatin protein.
And regarding, That is certainly my understanding as well. I've transferred magenta dye to a fixed-out (non-hardening) FB paper and the density is very good, but diffusion is terrible! The mechanism of imbibition though, relies on this pH gradient and not a mordant.
Speaking of which... you can buy thorium nitrate on eBay for fairly low prices. Not that I'm even considering such an endeavor..
Read the responses I posted and that Jim Browning posted.