So, you would need another Microsoft account; however, on Xbox One, you can have three Xbox Live accounts with the same membership. Note, also, that to play with your friend or a guest, you need to have two different consoles.
Yes, your guest can play without affecting your progress, in the unique condition of playing on a different account made for your guest. Othwerwise, if your guest plays on your account, it could affect your progress.
Let’s review: guests can play without modifiying your progress if they play on an another account (which doesn’t cost anything, like I said) - but then they have to start from scratch the game.
You would need another Xbox account, and it depends. If they do races then that will progress you towards the end as you do 4 races which complete a championship, which can unlock barn finds etc.
1.) Indeed, you have to create a whole new profile to start a “new game.” A bit of a bummer that older games had this, but they don’t do it more.
2.) Well in Singleplayer you go on “Roadtrips” from location to location and race at those locations. In Multiplayer they have something called an 'Online Roadtrip" that works similar but is a little different.
In Online Roadtrip everyone votes on the location and car class. Then everyone drives to each event in the roadtrip. Each event is a race or a game, etc. I don’t believe there’s AI in online roadtrips, its just whoever is in your lobby.
You can also just free roam with your friends. You can set up races in free roam by driving to the locations on the map.