The "weight of darkness" imo could work similarly to "shadow touched", it smothers your Light and thus temporarily drains your strength. And the Lamps reduce/remove the buff because they emit (possibly blighted) Light. I don't think the Bridge is a team test, it's not like Crota expects only teams. Ofc the Doylist explanation is that the gameplay mechanics are designed for teamwork. The reason Ir Yût doesn't start her Song right away might also be because it would also kill all of her minions. In the Book of Sorrows the 1st time the Death Song was sung it killed everything in Xivu Arath's Throne World. Crota didn't invent the Oversoul, his sisters did. They create it, tell Oryx about it when he asks, then Oryx told Crota to watch over what they were doing. Presumably this is when he learned it. Guardians can down his shield and put him on his knees with their bullets (well rockets mostly) because any gear they use is only a channel for their Light. Guns can't hurt Crota, the Light channeled through guns can. But in the case of Crota it can only tire him so to speak, it can't cause mortal harm. For that you need an Ascendant Blade, a weapon that has been steeped in Sword Logic for eons. But I don't think it's the sword alone, the Guardian wielding it is also channeling their own Sword Logic acquired through the passing of challenges and the slaying of ascendants within the Throne World. The Oversoul is not a physical thing, it's a spell that separates one's death from one's essence, it's a paracausal ability. The reason we can damage it is because Guardians are paracausal too.