I still have trouble visualizing how a 180 or 270 degree change can occur
in a
single rf cycle and be able to overcome the "inertia" (probably a
poor choice of words) of the rf circuits , feed line and antenna
system.

A sudden phase shift like that (within one RF cycle) only happens if you phase
modulate the carrier with a square wave, but then the tank circuit will even
things out, but will still be quite a wide bandwidth.

You don't modulate any carrier in any mode using square waves to be honest -
unless you pass that carrier thru a good filter after modulating it and before
letting it loose. You should low-pass filter the modulating signal before
letting it modulate the carrier - that way the phase change will be slow and not
sudden as you were thinking.

Clive