Even before you set foot in the lower area? In H1 (as far as I know), the waves of enemy only start spawning when you've got a certain way into the lower area.

: Regretfully, I have no idea what triggered this to happen. … I didn't do anything out of
: the ordinary as far as I can recall.

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

: Since I posted the videos I did go back and play the level without my save,
: and can't believe how few enemies there normally are at this battle.

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.

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.

: I figured that all the enemies that spawn through the corridors were
: triggered at once

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

: but its clear to me that I had taken down 3 to 4 times
: the typical amount of all the enemies intended to be in the battle, yet
: they just kept spawning again and again.

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.