If they wanted to show lots of dead bodies, why not have civilians and navy pilots and navy officers interlaced among the dead marines? Why is it only dead marines? Likely, because they are dead Marines who died in the manner that you'd expect a bunch of Marines to die. In direct combat.
What I'm trying to say is don't try to fit the cutscene around canon, because it doesn't. Just acknowlege that it contradicts canon, and you can either say it's an alternative canon or (as I believe) is not a part of canon. I mean you have the Hades and all kinds of other contradictions, so maybe its better to say it's not part of canon. Or you can say it represents another side of the story that the canon line doesn't show. Anyway as Dekker said, we cant really take it as Gospel anyway. I know I don't.
Personally, I don't think ground combat ever occurred, but I understand what I think the creators of the cutscene were trying to convey, that is the human loss due to the Shivans. I think that it was meant to represent a ground battle (that didn't happen according to canon). Its purpose was to help us visualize the human loss that a bug hunk of floating debris can't. and the fact that it's all dead combat troops emphasizes the power and the threat that the Shivans represent.
However, since my view really isn't in line with the topic, I'll close and say that I think that the Shivans are capable of fighting in 1G evironments, as the Iceni battle shows. "Some Shivans are amongst the dead" means that some of them were killed, but in the Hallfight cutscene the attack squad was able to kill at least one Shivan, so that doesn't surprise us. But when the Lieutenant said that it was a slaughterhouse in there implies that it was a slaughter of humans, not Shivans. This is also evidenced by his nervousness. If they were able to fend off the Shivans handily, he wouldn't be quite as anxious to get off the ship.