1) A 50/50 is when you have usually 2 options. An option that hits overhead, or an option that hits low. The point is simple, if they block low, your overhead can hit them, if they block overhead, your low can hit them. Making it a 50/50 chance, you guess the right option. There are other options of 50/50's, but this is the most basic.
2) A mixup and a 50/50 is pretty much the same thing. It's mixing up your strings and specials to make them guess the correct position to block. You can also ( and especially want too with Kitana ) use throws to mix someone up as well. This is a mixup, because you cant block throws. If they're trying to block your low or overhead option, and you throw them, they're going to get hit by it.
3) Not really a yes or no question. Some mix ups are referred to as "hard to blockables". This is when you can almost hit them low and overhead at the same time, making it very difficult to block both of them. For example, Summoner Quan Chi, and Cyrax. These type of mix ups are generally hard to block, ( hence the name ). Otherwise, most other mix ups are just making the correct read/guess.
4) Technically? Yes, in game? No, not really. Her b3 hits low, and her b2 hits overhead. Even though she has an overhead, playing Kitana, your opponent is going to block low against you, 95 % of the time. This is because her b2 is literal garbage. Its slow, and doesn't launch. Im also pretty sure its punishable. ( could be wrong, i almost never use it ). So its very big risk for very little reward. Pretty much the only good thing about it, is it is a hard knock down. Meaning you can end your combos with it to prevent their wake up/tech rolls. Her REAL 50/50 is when she has Xray. ( Which happens more then you probably think ). When Kitana has an Xray, she has access to a 9 frame overhead. So she can force her opponent into a mix up situation fairly easily. 2 good ways of doing this are: B14. This string usually hits Mid - Low. However you can do b1 - Xray. Which makes it mid - overhead. Your opponent is forced to guess which option youre going to do. The 2nd way, is doing b3 Xray. Not really a 50/50 as it will always hit low - overhead. But the transition is that fast, that it will catch anyone who isn't looking for it off guard. ( sometimes will still catch the ones that are looking for it ). Hope this helped, and if i was wrong in explaining anything, someone will correct me !