1.
ABD can book your flights to and from the US, but they are not included in the price of the trip. If you're paying for the flights with cash, then it's easy enough to book with ABD. If you're using points or miles, then you'd have to book on your own.
2. You don't need a TA, but it's nice to use one for ABD. Since you can't book directly through the website, there's a lot of back-and-forth communication with ABD, so having a TA can be easier.
3. ABD can add a
Disneyland Paris extension to the end of the trip, but it's limited where you can stay, and I think you have to purchase the highest tier meal plan. The ABD extension does include transfers from Paris to Disney and Disney to the airport, but you might need to call and see if what they offer matches what you wanted to do.
https://www.adventuresbydisney.com/destinations/europe/disneyland-paris/