3) When does online check-in open?

TUI flights
If you're flying with TUI, online check-in will open 48 hours before departure. You'll need to check in for your outbound and return flights separately. Online check-in for your outbound flight is available right up until six hours before take-off and, for your return flight, closes six hours before take-off.

To see if online check-in is ready for your flights yet, visit our Flight Extras website and enter your booking reference, the surname of the lead passenger and your departure date. You'll then be able to click the Check-in button if it's available. 

The same service is also available for the following airlines: 
Air Europa, Albastar, ASL Airlines France, ASL Airlines Ireland, Evelop Airlines, Germania, Norwegian, Travel Service, Volotea.