Well, Navpi, the way I see it is a bit different.
For facts, or rather for a relatively comprehensive analysis, the Airports Commission did the job, not perfectly, but to a reasonable level given the amount of uncertainty and change which is out there in the world. Also, Davies and his Commissioners reached a judgement on a set of recommendations. Now eighteen months later (much to the disgust of several on this forum) and with a lot more behind the scenes activity the subject has been moved along to political decision time.
What is the nature of the decision? It is that the Government will support a set of actions
A B and C intended to lead to outcome X. This is bound to be subject to a debate and motion in the House, probably it will be crawled over by the Treasury or Transport Select Committee and there will be many JRs just like HS2. That's the system.
By the way, if it is Heathrow NW and £18bn is to be financed ultimately from air travellers via airport charges, that component of the funding has nothing whatever to do with the Barnett Formula. Surely what the Scots have primarily in their minds is that from their point of view strengthening the hub does something for them while expanding P to P capacity in the London system does nothing for them. Of course that is only one of many dimensions in this decision.