Hi, uncle_joe!
Yes, Major is the real deal ironman-randomness-just-like-in-real-life mode. It's also the mode that's automatically activated in tutorials and multiplayer games. No advantages for the AI (If there's a big request for an "Impossible" difficulty level, we'll make one in which the AI can use veterans and ammos

).
The "Captain" mode is very similar to the Major, but with a slight adjustment to the dice rolls. It really IS slight (mostly to avoid bad runs of 1s for you and 6s for the AI). The old not-ironman-mode randomness was more similar to the Liutenant mode, which gives you a much softer randomness.
You're absolutely right about the protect/survive missions: adding turns can actually make things MORE difficult. We even thought about it, but somehow ended up with adding the turns in all missions anyway (ouch!). That's bad. I fixed it so that turns are NOT added in survive/protect missions (we still want you to be able to complete the secondary objectives / grab the loot, after all). We'll put this in the new patch.
Thanks for pointing this out.
Roberto