Ya, never did set a foot in the lower level - they started to load once I nearly cleared out the upper level. This also means they loaded before I want into the hallways which slope down to the lower level. This did strike me as strange, which is why I thought if i stuck with it and plowed through what was down there all at once I'd be done. After killing endless covies and going back up a level i was dumbstruck that it looked like i had done no work. I thought something was wrong before, but at that point I realized just how crazy it was.

: Did the Marines get wiped out in the first room? And can you recall anything
: of the sequence of events after completing that battle?

All the marines died in the first room. Nothing really struck me as unusual after that, pretty sure I encountered the couple of standard small teams in the corridors (2 grunts at first, then some elites and grunts) before reaching room 2. I do save the camo for use after the first room is cleared out including the hunters, so more than likely I assassinated most of the enemies in the corridors.

Oh, I also never went down to the locked door to trigger the discussion with Cortana and the would be marines. I was trying to get another checkpoint so I walked all around the areas which were clear including the locked door area and Cortana finally chimed in.

: I've already been giving myself a quick refresher. On Legendary I let the
: waves collect, and counted 3 Elites, 6 Jackals and 14 Grunts. Obviously
: nothing like the numbers you've been seeing.

I can distinctly remember killing at least a dozen elites in one of my attempts, probably more, and obviously plenty more were still alive.

: Do you still have a save with this freakiness? If so, that could obviously be
: useful in trying to figure out what's going on.

No, I gave it up, but I remember exactly what happened at that saved checkpoint since I did it so many times. I was carrying the AR and sniper, had a total of 6 sniper rounds left. I had 3 stickies and no frags. I only had one bar of health, but also had not used the medkit at that point (i backtracked and got it for most of my videos uploaded). The automatic door to room 2 flashes and starts to open right as the save loads up, I don't really remember killing anybody out there, but all that remained after the checkpoint were a total of 4 jackals (2 tucked in the far corner) and 2 elites on the upper floor. I believe that means I must have killed a couple grunts before the save, unless they just didn't load, not too sure on this one.

Part of me regrets not keeping the game saved, but it just wasn't very fun given the ammo I had remaining coupled with my health and the need to backtrack for the med kit. A few frags would've been nice to toss around too.

: There aren't very many though, if you're talking about the enemies between
: the first and second rooms.

No, not between the first and second rooms, but all the enemies that spawn in the hallways connecting to the second room's lower level. To help clarify, this includes the hallway that leads to the main hanger in the next sequence of the level.

: Did you have to kill some to get new enemies to spawn, or what?

: By the way, in your original post you mentioned losing a pistol, but in H1
: there's sadly no pistol available on this level. So I guess that's one way
: MCC is different. I'm surprised to hear it though.

Ya, there is no pistol, realized that when i restarted the level. I was attributing my utter failure to not having the pistol since I never recalled this section being so difficult, no it's clear that my firepower wasn't the real problem.