"What's up" when used as a greeting follows the pattern of How do you do. It may appear at first glance appears to be a question, but it is in fact stated and given as an idiomatic set phrase, the correct response to which is to repeat the question:
John: Hey Matt! What's up?
Matt: Yo John! What's up? How've you been? I've not seen you since Monday!
"What's up" is an informal greeting, and should only be used in informal situations. If in doubt, do not use it to greet people that have not greeted you with "What's up" in the past. For more formal situations, use hello or hi.
No.
Firstly, "thou"/"thee" is not modern English outside of dialectal usages (which I believe is its context in Uncle Tom's Cabin).
Secondly, you're right about the parallels to the two pronouns in Spanish, but "you" is the polite, formal option. Actually, Spanish is a bit more complicated, because it has (I understand) familiar and formal versions for both singular and plural. English is more similar to French; in both languages, the singular ("thou", "tu") is also the familiar, and you would use the plural ("you", "vous") to an individual if you wanted to be formal.
(Related reading: "T-V distinction".)
Thirdly, having said "no", let me now change it to "no, except when it is". Thanks to "thou" falling out of usage, most people are only familiar with it in old works, especially the King James Bible and Shakespeare's plays (both Early Modern English, from around 1600). The former, in particular, has caused "thou"/"thee" to be associated with formal situations, because God is addressed as "thee" (because it's singular, not because it's familiar).
However, that only applies to people affecting old-fashioned usage. They are quite likely to use "thou"/"thee" as a formal pronoun. People who use them because it's part of their dialect will, I'm sure, be using them with their original (singular and familiar) meaning.
Best Answer
Good evening. Good night is definitely for good-bye's only, not as a greeting.
As to whether you'd use "today" or "tonight", that might be a convention of that profession, or that particular set of people, so I don't think anyone can give you a single good answer for that.