Anyone who has tried to operate multiple dipoles in parallel will know
that without heroic effort this is a false premise, nevertheless, the
practice continues.

Huh? Never had a problem here. But I don't "stack" mine like
some people do. That's what causes the coupling problems.
If you parallel feed two dipoles at right angles, there is very little
interaction at all. I spread mine apart as far as possible.
Problems are rare, and easily cured by just moving a wire
a few feet if it happens. It's the ones who tightly stack them with
short spreaders, that have the nightmare tuning problems.

Multiple radials of different length are
often operated in parallel because it is thought that there is then a
set that is optimum for each band.

Well, there is a set that is tuned for a certain band. They won't work
worth a hoot if they aren't resonant. But this applies only to
elevated
radials. On the ground, I agree...measuring is a waste of time.
Doesn't matter if the wire is insulated or not. Will act much the same
either way.
MK