1. If it is a subject line test, then it will pull the existing subject into the test to be the "champion" you are validating. You would then specify another subject to use as the "challenger." If it's a whole email test, then it will simply test the "challenger" email you select against the "challenger" email, which is the email you created the test from.
2. You are entering the subject yourself, so you will know what it is. If you are asking because you want to use a token then the answer is no, you would need to know what the token name is and what the value would be ahead of time. You could just send a sample test to yourself if it is necessary for you to test it for some reason.
3. Yes, once you setup the champion/challenger test it will ensure that anytime that specific email is sent out, the test rules are followed. In other words, it will keep sending some % the champion and some other % the challenger until you declare a winner.