I think you will find that as an associate member of EASA the ECJ red line would/could be crossed. Any dispute within EASA of those countries, (full or associate members), signed up to it are subject to the ECJ.
I cannot see the hard line Brexiteers being happy as an associate member with no influence and subject to the ECJ.
Over the 10 years it took to agree Open Skies the UK had a major influence. It still does within EASA.
If agreement cannot be reached then UK airlines are in danger of being excluded from EASA and the Open Skies Agreement. That is why easyJet and Thomas Cook are moving their HQ's to Europe. Furthermore the major EU airlines are badgering their governments to exclude UK airlines from open skies because of the benefits it brings them.
Below is a list of possibilities if the UK is excluded from EASA and the Open Skies Agreement.
The UK would have to negotiate sovereignty over its airspace via ICAO, IATA and the 27 EU countries
Probable very limited “freedoms” of the air within the EU
Traffic rights given by bilateral agreements
EU countries traditionally protect national carrier and limit/prohibit competition. UK would probably do the same.
Use only of designated airports
Often single airline designation between airports leading to increased fares due lack of competition
Limited frequencies / capacity
Double approval for fares leading to increased fares
Pooling agreements between airlines leading to increased fares
Most airlines state-owned with independent airlines facing unfair competition as governments protect their own