I stick to the brother theory, it makes a lot of sense in that context.
It would be daunting if you weren't Da Vinci. I don't believe he was alone, I don't think the workers made the trip back so to speak. They found at least two different human bones 180feet below, in an area never searched. They ran a DNA test and they came from Europe and the Middle East. Templars were from all over, they didn't have a national allegiance, they died for the cause.
Da Vinci learned from someone, I don't feel this was all him, btw. His "sect" is responsible, he just made it all come together and what we see now is his doing. IMO.
Thanks and we will never know the truth, so let those wild far fetched theories fly!