These are al extremely rhetorical questions, but a non-EU country can subscribe to the Single Market for air transport outwith the EU - for example Norway and Iceland. I could very well foresee an iScotland - if it were pursuing eventual membership of the EU - negotiating a place in the EU 'acquis' as regards aviation, and benefiting from both the liberalised internal market and e.g. the EU/US bilateral aviation agreement.
Negotiations would be in order but not nearly as complex as some of the other stuff they would have to sort out.
But dinnae fash yersel', it's not goin' tae happen
written while DJ6 was already posting his excellent answer