1. It's just Ito. If it's Itou, I would put it down as Itō and they are not the same. In regards to whether they are kanji or kana, looking at the information that I've collected, most of the women named Ito have it written in kana (most of which is in katakana) with one of them having her name written with the kanji 糸 which means "thread."
2. Assuming Mitsu and Hikari/u have the same kanji, I can't really explain how the shift occurred but I might assume that parents of Generation Y, millennial and post-millennial babies wanted to give their babies a name that doesn't sound outdated to them (with some exceptions like
Haru, Natsu &
Yuki). I didn't find any Mitsu with the kanji that is the same as Hikari/u but the closest I can find is 美 meaning "beauty/beautiful" mixed in with つ, the hiragana character for 'tsu.'
My PNL:
http://www.behindthename.com/pnl/149249