Like I said, you have much easier and simpler options in the continued denying of all that has been placed before you. If you wish to continue to believe that the Adams Onis Treaty contained references to the two undeniable sources of wealth in the Texas region that your author allegedly read about in grandpa Risque's library then that is your choice, but, I have never seen or read such reference in all of my review of this treaty, nor do I know of anyone else who has. Again, if an alternate truth is ever to be discovered then no doubt folks are going to have to detach themselves from all of the redundant romance and lore surrounding the publication. Here is yet another interesting bit of information for you, should you be so inclined to take interest in it, which I seriously doubt.
"The U.S. claim was for return of, or compensation for, slaves that were in British territory or on British naval vessels when the treaty was signed. The Treaty of Ghent article in question was about handing over property, and the U.S. claimed that these slaves were the property of U.S. citizens." Britain did eventually agree to compensate for the loss of these possessions. Also as part of this treaty, a specific territory along the Rocky Mountains was also a source of hot issue, especially for the Americans, this also being an area of the same general hot issue during the Adams Onis Treaty. So, you can either continue to keep things as romantic and as simple as possible or you can opt to entertain those things that are far more complex and complicated and documented. Up to you, really.
At the center of all of this was James Monroe, both Issac and Edward Coles having served as his personal secretary. in 1829 Joseph Bonaparte sends letter of inquiry to Monroe from Richmond Virginia regarding payments for services rendered, in reply Monroe does not deny that services were rendered but only that "promises" were made to make payment. The documented evidence and connections before is piling up so heavily on all sides that very soon now you will no longer have view of any horizons.

Perhaps now you might be able to draw upon clearer line as to why Monroe really sent George Graham into the Texas region, not because of just one treaty, but because of two.