3) Can I upgrade my seats when I check in online?

Yes, if any seat upgrades are available, you'll be able to see them on the seat assignment page whilst you're checking in online.

TUI flights
How to upgrade:
Upgrades like Seats with Extra Space, Seats with Extra Legroom, and Premium seats, will be shown with different colour codes on the seat plan. Select a passenger on the left-hand side of the page and then click on the seat you want on the plan. The total price will update with any extra cost.

For a bit extra, you can add things like top up baggage, champagne and chocolates, or space for any sports equipment you'll be carrying. Simply click on Select Seats within 90 days of the date of your return flight to upgrade your seating options or add on any extras. 
Please click here to check in on line.