BetterTimeWarp's MonoBehaviour needs to be run after KSP's TimeWarp, which seems to have an unspecified DefaultExecutionOrder of 0. As 100 is bigger than 0, BetterTimeWarp is now called after TimeWarp, but before Unity physics engine update.
Lossless physics were not running on TimeWarp.Modes.LOW, as it was explicitly checked against.
No need to store maximumDeltaTime to disc (on GameSettings.PHYSICS_FRAME_DT_LIMIT), if it's actually assigned each frame.