Speaking of quadrature demods, wouldn't best performance be had by
using a 23.3ns ) wideband delay instead of a single or
double-tuned LC. I just found one with 30MHz BW...

Probably not, unless there's something I'm missing. You don't want a
wideband circuit here -- the quadrature detector takes advantage of
phase shift through the passband. Basically it uses a mixer to compare
a signal with a phase-shifted version of itself, with the output being
proportional to the actual phase difference. With a single-tuned
circuit, the 3 dB point corresponds to 45 degrees of phase shift, and
that's where you center the quadrature coil.

-- jm

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