Answering 2 items:
A shorter wavelength (playback) laser would allow a wider bandwidth on CDs.
(IIRC, Louis Dorren stated that the Vinyl LP can store frequencies up to 90kHz)
I have an HD radio, 1190 AM used HD (HD was in 1180 and 1200 frequencies),
at night, 50kW 1200 AM WOAI in Texas interfered with the upper sideband.
One FM HD station rebroadcasts an AM talk radio station but sets the data
rate too low resulting an "underwater/burbling" sound.
I have listened to a few of the FM HD signals thru my Dolby Surround system,
generally FM Stereo content doesn't have much surround content though.
Kirk Bayne