Yes the definition is moot up until 12 or 6. That's been a number of people's point all along.
It's moot because the 30/60 doesn't cover low powered air guns. That's why Page 13 para 2.46 makes it clear as guidance that if the gun isn't low powered it does. That's what the document is for, guidance.
In Bighit's link the judge said there is a circular argument which doesn't work because there's no definition of when an air pistol is not an air rifle until it's over the limit and then it is suddenly a pistol despite it not being one before. This issue comes up in a number of areas of the Act, like taking air rifles off ticket because they are no longer air rifles when they are on ticket. It's just the way it works (or doesn't). The reality is that just like other areas it doesn't affect anyone who hasn't already established a good reason for the law to be looking at them beforehand.
There is no definition of what an air pistol is. As I said before it is generally accepted by those concerned enough to look into it that it would fall upon what a reasonable idea of what a pistol was. Sawing the butt off may or may not define it in the same way adding a folding stock would or wouldn't either.
No.
Long barrelled revolvers need not only a barrel length of 30cm but also need an overall length of 60cm because they can't rely on just one aspect otherwise they'd be sect 5. That's why they have rods welded to the back. If they just needed one they wouldn't bother with the other. It's still Either/Or. If they have just one they are sect 5.