1 - One way up to 3 segments
2 - Yes, they are complimentary. For an EXP you get an e-mail (sometimes) from t-100 hours to 15 min before departure. You can use the AA app at t-4 to see where you are on the list or if you cleared.
3- You use them for a companion or when you drop down from EXP to PLT or GLD. I have had 44 upgrades in my account for years. They don't expire.