Why Does Toneri Want Hinata?

Evidence. It is revealed that Toneri is in love with Hinata and desired to use her to revive his clan (But in truth, he is very naïve of what love really is). Toneri called Hinata the "Byakugan Princess". Toneri asked Hinata to become his wife.

Why did Hinata go with Toneri?

Hinata chose to left Naruto and went with Toneri because.. Hinata and the others doesn't know where Toneri's castle in which Hanabi is in. She did that to save her sister. And to learn what other future plans Toneri intends to do.

What happens if Toneri married Hinata?

yes they will have the tenseigan. The direct Otsutsuki descendents of Hamura all inherit that ocular jutsu. Just like the Hyuga inherit the byakugan, but because of Toneri's DNA, Hinata and his children with be more powerful and will inherit the tenseigan.