When half the flat portion of a balanced feedline is against any material then that side of the line's signal being transfered is attenuated during transmitting.
100% agreement. The issue of what's within a few line widths of the balanced line matters. I wouldn't call that a "radiating feedline" per se, but rather it's a condition of having an EM field as an inherent property.
What I am referring to is the "vertical polarization component" many claim a G5RV to have due to "radiating feedline". This is not possible unless something about the antenna is imbalanced. Yes, there may be a small amount of common mode due to being immersed in the EM field of the antenna but it's not being "generated" by the feedline.
> 3/2 wavelength with respect to what
The design frequency.
> he and everyone else presses the hybrid vertical horizontal radiating G5rv antenna into multi band service.
Right. As has already been mentioned it's a 20M antenna and happens to "load up" on other bands. If it meets ones expectation, then it's a "good" antenna.