So... I'm confused. A friend of me owns TSW2 but not TSW3. Would he be able to use "TSW2 Compatible" content or not?
Yes they will, but if they're "buying from scratch" today TSW2 is no longer available so steam cannot link to it as a requirement
Yes and no. Your progress on journeys, timetables, scenarios, liveries and scenarios created using cc will transfer over and whatever you subscribed to. Some achievements will transfer over but not all. In order to get 100% completion you have to redo all tutorials (most are bugged and can't be completed since the port) and scenarios. You can always restart from scratch if you want but if achievements don't bother you, you won't have to worry.