You're absolutely right about the mouse sensitivity option. In general, it lacks of UI, settings and options.
Regarding the offset between the crosshair and the shot, I experienced, previously, some issues with mouse capturing with the Web version (in fact there was a slight offset between the cursor and the mouse position). I thought it was solved but there may be some scenarios in which the bug reappears . Did you play it fullscreen? Also did you experience any lag or framerate drop during the game?
Anyway, thank you for your feedback!