The AID delay compensates for length of antenna cables, antenna position, gear geometry and (typical) flare angle to ensure that the RA reads zero at the moment of touchdown.
With regards to your answer 3 whether in a calm lake/surface water will the radio altimeter give correct height over water, may I ask any reference to that for b777?
Look at any landing video and see how much lower the bottom of the gear is with respect to the lowest point of the fuselage (or engine cowls if closer).
The Rad Alt antennas have a conical beam (not a pencil beam), but the system will use a point on the ground which is closest to the aircraft for display purposes. This is why you still get a reasonable idea of height above the terrain when the aircraft is banking.... unless you decide to bank at 90 degrees... in which case the Rad Alt display will disappear. This conical beam means the RA may use the top of the fin of the other aircraft. This reduces your 80' difference somewhat, as would the relative position of the pitot static sensors with respect to the bottom of the wheels.
Baro vs radio altimetry