Extra Avios availability is indeed only in Y - which can be very useful on short-haul in particular: I very rarely struggle to find availability for European redemptions in Y.

The 'double points' thing works across all classes and can be an excellent way to save cash - typically to force double points redemption on short-haul Y where there is peak demand and high cash prices...on those redemptions, even 'double' points doesn't dent the Avios balance much and you can save a small fortune. I've done for Munich during Oktoberfest, for example - saving literally £700 on the cash price for Y...

The double points thing would work for First, but at those redemption rates you'd need a serious Avios balance to make it worth doing. You also cannot combine a double points redemption with an Amex 2-4-1 so it would quite literally be double the normal First redemption rates (plus taxes).