Not if the cinematics are synced to a global timer instead of the frame rate. Then when the frame rate is too low the game skips some frames to keep up.

No, the game uses frame delta times rather than a set tick rate per second.

(In general it's how most games already work; it's just that it's normally convenient to set it at a fixed rate to make some things like physics more deterministic, and it gives a nice target performance/budget).