Check out Themeparks.com for pretty accurate hours of operation. A couple weeks in April should be open till 10 PM.
I don't think there is a late day discount. How many days you're staying and how soon you may return to Universal will help influence what type tickets you may want to get. You should know also, unused days don't expire!
If you're staying on-site and using Front-of-Line Hotel key card, you can get a lot done between 4-10 PM!!
Hope this helps!