Question about the Tenseigan (Spoilers Maybe)

So after watching The Last, I had so many questions about the Tenseigan.

1. What is the relationship between the dojutsu and the big sun-like ball in the center of the moon? Other than their name
2. Why does it revert back to the Byakugan other than for the sake of the story. Does that mean the Rinnegan can revert back to the Sharingan if certain conditions are met? Serious answers only please. Thanks fam.