The book is 400+ pages, contains all authentic narrations, all weak narrations, all quotes by historians and genealogists. Too much to translate, I only quickly translated specific key words from authentic reports (for the most part) which clarify the truth of the matter and answer your questions.
The girl was unaware of being his wife because she's a kid, her father later explained to her. As for sending his daughter with her two brothers to be married as soon as Umar said "I accept", then you must know that these folks had really close ties and were tightly nit community of early Muslims. In other words, Ali was not sending his daughter to a stranger, recall the words "Go and marry her off to your uncle."
What you understand from the narrations is correct. Ali knew for a fact Umar would most likely accept regardless of her age or beauty.
As for those of prophetic descent in our days, there's millions, mostly Sunni Arabs. Half my family are descendants if Musa bin Jafar and some of them are atheists, not to mention 5 Christian families from Hashemite descent in Lebanon. If you marry into them with the same intention of Umar you're rewarded, if however you're after status or worse in case of Shia (Khums) then you will get punished for a behavior of ignorance and plain racism.
Our conversations would be a lot quicker and better on WhatsApp bro, trust me :p