I'm not a necron player, but I'm assumjng dynasties are just a keyword, so I would say;
#1 yes,absolutely. That is exactly the proper use of detachments
#2 yes and in fact it doesn't need to be an hq, just a character
#3 yes, you can use the rules of a single keyword and rename it to something unique. Just like marines do with successor chapters. You have to take all the rules involved with that keyword, good and bad. You can't cherry pick.
Hope that helps.