This is just my theory, but I think that white fluff represents Jack's "rational" thoughts and his blue doppelganger represents his unsuppressed subconscious. Jack is fighting both, he's being irrational because he could just leave Ashi (who has proven herself willing to kill him at all costs) and escape, while also…

Wait, Automata's OST is better than Nier's?

I believe the implication wasn't that Steven and Lars actually switched bodies so-much as Steven was just projecting himself into Lars's mind while he was sleeping; he essentially overwrote Lars's brain for a day.

For me, Steven's generally upbeat/one-note way of speaking serves to make the moments where he expresses something other than optimism all that more poignant. Hearing his voice break as he begged Pearl to tell him what he did wrong in "Rose's Scabbard" damn near broke my heart.