Oak trees don't have tap roots, but put out their feeders on and just under the surface of the ground. They are fairly fast growers that will grow over and around things such as fence posts and other things that don't move away. I think the roots in the second picture are just growing over the roots of the tree on the right. It looks very interesting, but I don't belive they are results of a graft. Tree grafts done by the Spanish (and anyone else) would be in the crotch of a limb or ON a limb or out of the side of the main trunk when it's young and tender. Root grafts are usually done by cutting the trunk off of a hearty young sapling and grafting the new piece directly on it. The best example of this technique is the hi-bred rose bushes we all buy these days. That's one reason why trying to "root" a cutting from one of those is so hard. The plant is a weak one that is using another's root....from the root head on down (all of 3 or 4 inches.).
Don't know about that graveyard tree. There have been old treasure tales of booty being buried in graveyards with a headstone naming a fictious person. The only way to check THAT out would be to back check every name in the graveyard which would be a really tall job to do. Especially with so many old stones that are either missing or unreadable. That can get irritating, too. One of my other hobbies has been genealogy.......researching my family history. Here's a really good tip for you. Use the cover of being a hobby genealogist when doing any treasure research in a library, courthouse, or museum. Those folks will bend over backwards to help a genealogist, but will stonewall a treasure hunter because all of us treasure hunters are vandals and thieves. LMAO