pndrev: New maps - are those new world textures, terrain meshes, complete new maps?
Yes to all.
pndrev: Do they include updated / new campaigns and skirmishes?
Yes, for both the original terrains and the ones included in the mod.
pndrev: What 3D cockpits are included, are both EEAH and EECH cockpits updated?
Cockpits are included for the Comanche, Hokum, Apache Longbow and Havoc. The latter two are greatly enhanced from the original EECH; the former two have a few differences, mostly in the HUD.
pndrev: Is general view distance increased, newer models?
Not sure.
pndrev: What do 'other things' encompass? New flightmodels, AI routines, campaign engine?
Flightmodels, AI routines and the campaign engine are tied to the game executable, and so are available even without the mods.
pndrev: It a whopping 550MB+ download that apparently uses an older version of the executable than on GoG.
The reason it uses an older version is because it was created before 1.11.1, and the allmods installer hasn't been updated (yet) to include the 1.11.1 exe, since all you have to do is apply the Allmods.exe, and then re-update to 1.11.1. I'd say that's pretty easy to do.