I'm have two working theories about the tree with the mysterious symbols on it next to the Mysterious Hill Home.
To me the house is directly indicated by Algers Helgerson when you meet him in the Epilogue as the place that the new Chelonian Master comes from. You find a suicide ceremony at the original location from the quest that Mary sends you on.
One theory is that the texture change related to this tree is related to the long distance textures carrying the text but that an event was removed so when you get close had been cleaned up - simply an artwork issue on final release that never got corrected.
Understanding the text though I have tried a lot based on the theory that it reads Leader X with Leader doubled and back to back and X as a reference to the map symbol for dead.
I've tried all sorts of placement of both the new Chelonain master (with the shell) and Algers (he's the one marked Chelonian - he says in St Denis that he will be joining the ceremony IIRC) - round the tree, back to back with a little lasso dragging to get them that way and then using a fire bottle.
Also working on some other area I brought two snapping turtles up (assuming it might be a Turtles all the way down ref) and did the same thing.
One thing I can say that on occasion there is a distinct boom when the fire bottles go off and the tree turns sort of mesh and wierd but I think either it is a glitch or if there is something to be done I don't have the complete picture.
I've tried various extra things like doing the jump with the Chelonians and then completing.
Haven't actually tried lugging up the Chelonian Master all that way manually with two horses carrying 3* Snapping Turtles to make it a 3 layer turtles all the way down. btw last time I rode the two 3* Snapping turtles up I discovered one had a green belly and one had a white one.. probably nothing at all but you end up thinking like a conspiracy theorist when trying to find secrets in games.
I might leave it here and just go and research Spider Webs since I'm late to the party on that so I'm just leaving these observations here in case anybody else wants to take up the baton.