There seems to be an assumption by some posters here that
a mobile antenna must be electrically 90 degrees in length.


Must be? Must is only death and taxes!

Resonant monopole must be 90 electrical degrees and resonant dipole must be
180 electrical degrees long. But that "must" does not apply to antennas as
chosen by users. You can use light bulb, screwdriver, bedsprings, wet noodle
or any piece of RF conducting material of any length, shape you like, just
question how good of antenna it is.

We used 90 deg. monopole in discussion to show typical loaded or mobile
antenna in order to avoid detours to la-la land (cases when current can be
and is equal) and to stay on the subject of current magnitude along the
typical loading coil.

73 Yuri, K3BU