A1: TruckersMP doesn't change the job market. It's the same as it would be playing single-player. Unless of course you use World of Trucks contracts to get the same jobs as your friends.
A2: TruckersMP uses your single-player profile when online therefore any progress you make on SP will impact MP vice versa.
A3: If you accidentally hit someone then apologise immediately, if it's clear that it was not purposeful or because of lag etc there will be leniency.
A4: You can record your gameplay at all times but it's not really necessary. If you want to keep your file size low on recordings just use Shadowplay to record certain small clips.
A5: Same as A2.
A6: World of Trucks (https://www.worldoftrucks.com/en/) is an online platform integrated with ETS2/ATS which allows you to track your progress, deliveries, achievements and more.
It's more commonly used to get the same job contracts as your friends. However, your speed will be limited in-game hence why most people use the standard 'Freight Market'.