3) When does online check-in open?

TUI flights
If you're flying with TUI, online check-in 72 hours before your outward flight from the UK and Ireland. You can check in 72 hours before your return flight too.
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.