The two-tone chrome separation is the right call. Having the sidebar at neutral-2 vs content at neutral-1 creates a clear spatial hierarchy without needing borders.
Agreed. And the inset-shadow approach for depth keeps it from looking like Material Design's "floating cards" problem.
Spring physics for all interactive motion is the right default. The 600/40 config feels snappy but not jittery. Messages at 500/35 feel slightly warmer which suits the chat context.