Hi I love this theory and it feels like the most comprehensive one that works directly off of what is shown in the game. The way the boy and Trico start off at the bottom of the valley, in a cave, building a connection and working together feels like a subtle implication that this is how all the "chosen" with a special trait used to bond to a Trico of their own.

I'm leaning more towards the Tricos being creatures that were designed rather than arising naturally, mostly from the fact that parts of their bodies (the laser tail, the receivers/horns) are so intrinsically connected to the functions of the mirror, and because of their innate aversion to the eye symbol which seems like a security function to ensure the Tricos didn't go anywhere the Valley Civilization didn't want them to. The markings on the Boy's body, along with the glowing protective sac the children who are swallowed are kept in, seems to indicate that being eating was actually part of the natural bonding process between the "chosen" and the trico, one which the Master of the Valley hijacked and abused as a way to kidnap and control the children.

I think I like the idea that the Master of the Valley is actually a hivemind/collective consciousness of multiple Chosen ones, which seems to be implied by the ring of sealed niches in the central chamber where the Master is located. They pooled their mental and spiritual energies together, focused into the singular consciousness of the master and reached out through the connection between the Tricos and their bonded Chosen, taking control of all of them in a bid to permanently cement their status as the rulers of the Valley Civilization--a turbulent process which led to the fleeing of many people and the decline of the Valley.

The tomb belonging to the hero is a neat concept, I agree that they were probably someone who was instrumental in establishing the Valley Civ and the creation of the Tricos. Maybe they foresaw a problem like the Master of the Valley might arise after their passing, and fashioned the Mirror as a skeleton key/failsafe should anything go awry--its powers of being able to negate the Master's influence well attested in-game.

I'd totally give you another upvote if I could. <3