1. Options are 1850, 1900 and 1950. You need to play in 1850 first and once you reach 1900 you can forever start games at this date.
2. Until you fill your entire hard drive.
3. I don’t know. But I highly doubted you need to start a new game to use a new DLC for example, or a vehicle added by mod. Its just not how game works. Not TF or other games.