When it comes to the bilateral, please read Article 2 - grant of rights. That contains the secret sauce of the bilateral and sets out who can do what. The rest of its content all supports that clause.
It’s for Jet2 to explain why they have set up shop down the road. My guess is that they will have looked at a postcode analysis of their customer base and recognised that they are weaker in the Liverpool area, where passengers are flying with Ryanair and easyJet (increasingly active in the holiday market) from Liverpool. If there is a part of the market you aren’t attracting but believe you need to, you go set up shop where you need to be to capture it.
What I do not “get” through all of this is the assumption that there is some conspiracy theory in high places to do Manchester down. It’s just bizarre. And Skipness makes a good point about growth coming from other areas, just not transatlantic right now.