Do later effects which explicitly increase the creature's land speed (Expeditious Retreat, Barbarian's fast movement, Scout's Fast Movement) also add doubly to the Half-fey's fly speed?
You're silly. That's silly.
Fast Movement (Ex): Starting at 3rd level, a scout's gains a +10 foot enhancement bonus to her base land speed. At 11th level, this bonus increases to +20 feet. See the monk class feature, page 41 of the Player's Handbook.
Fast movement does not give you a new base speed. It gives you an enhancement bonus to your base speed. Which means this is like asking if these effects stack with Haste.
They do not.
So, why would a half-fey catfolk with a now natural fly speed of 80 suddenly be granted +60 ft to it's fly speed because of, say, Haste being cast on them? Oh wait, that's actually a +90 ft bonus to it's fly speed because:
All of the hasted creature’s modes of movement (including land movement, burrow, climb, fly, and swim) increase by 30 feet, to a maximum of twice the subject’s normal speed using that form of movement. This increase counts as an enhancement bonus, and it affects the creature’s jumping distance as normal for increased speed.
Or just +60ft if you consider that to be trying to stack the same bonus type from the same source.
Regardless, none of these effects give the character a new base speed, they only grant typed bonuses to existing speeds. It's fairly clearly said in such abilities, spells, or class features; whichever.
I could see adapting the Barbarian Fast Movement entry from Stormwrack to apply to fly speeds rather than swim speeds:
Quote from: SW p. 48
Fast Movement (Ex): Barbarians who possess a racial swim speed can choose to apply their fast movement bonus to their swim speed instead of their land speed. The choice must be made when the character gains the class feature and cannot be changed later. This benefit still applies only when the barbarian is wearing no armor, light armor, or medium armor and not carrying a heavy load.
Edit: Stormwrack actually has the exact same entry for Scouts (changing all references of "barbarian" to "scout") on page 51.
I, too, could see that as being entirely agreeable considering there is existing precedence, as you've shown, for such options for alternate speeds besides Land Speed.
What is the fly speed of a Half-Fey Catfolk with 3 levels of Scout?
80 feet. Barring a DM that agrees with Stormwrack's precedence for allowing Fast Movement class features to boost other speeds in place of Land
Edit: On read-through after posting......I kinda sorta maybe apologize for apparently coming off as condescending. At least I think I did...Oh well, not doing it over