Well, having purchased it a month or two ago...
1. most definantly, unless Mr. Decanillo has died, which is impossible, no mere mortal could develop a full scale game on his own (he's up there on olympus somewhere with chris sawyer and will right im sure)
2. not necesarily, the game (as of version 4.5) is fully functional on it's own and doesn't use any player made fixes. Although the SDK is powerful, so there are neumerous mods, I'm sure someone can get you the link to grouper or whatnot, that should have a bunch of the player made mods.
So where's the obligatory game video with topgun music?
In light of the fact that the campain is giving me a hard time (see the thread Can't Load a Mission in support forum), I was wondering if there are any sites where I can download more single missions. The few that come with the game are fun, but I need more as long as the campaign isn't working.