Ah. Well, in NONE of the coop maps I played were the AI set at anything but Red then. I guess both NL and the 3rd party guys were too lazy. Granted I stopped playing the game about a year after release...
Ok, the helos fly more like real helos than they did in JO. In JO you could literally fly it with one hand on the keyboard and never crash no matter what. In ArmA you have to pay attention or you could crash VERY easily. You also seem to get shot down by a MANPAD or Shilka quite easily if you don't stay low and use cover.
ArmA, and OFP before it, have respawns as a mission-adjustable thing. Some missions indeed give you a single life. Some have so-called group respawn, where for example if you have 4 players in a squad of 6 (2 AI filling in) you can die and respawn until that squad is depleted, then you will stay dead. So in other words the first 2 people to die (or 1 person that dies twice) gets to respawn. After that, you're done. Yet other missions offer "normal" respawn where you just start back at base or wherever every time you die. Some let you respawn instantly, and I've seen delays as much as 60 seconds (a good way to discourage going Rambo without permanent punishment for a mistake).
As for how accessible the map builder is or isn't, I can't comment too much as I've barely touched it, but I'm not a map creator type. However, there are literally dozens of missions coming out a day.
Like OFP before it, ArmA comes with set maps. In ArmA's case, 2 islands, one of which is far larger than the other (the large one is where the SP game takes place as well). It's pre-filled with towns, buildings, etc. Mission makers place friendly and enemy units and you can make field bases with tents, bunkers, and sandbags and such anywhere you want.
It is possible to make new islands, I remember a few that came out for OFP, but it's VERY time consuming, and the terrain on the existing islands is so varied that people tend to stick with it.
There are other game types than coop, such as CTF and such, but I never play them.