Re deceleration; DECEL light. Surely this only indicates that the system is engaged (and working), and presumably ‘without a gauged’ measure, the system does not indicate how much of the demanded deceleration is achieved?
Actually is does, the 'decel' light tells the crew that 80% of the commanded deceleration rate is being achieved.
However, your point remains very valid as there is no way of differentiating between the deceleration provided by the reversers and that which is being provided by the brakes.