At that first stage of exporting out to a WAV file, it is a lossless export
Here we agree.
but you have imposed lossy things upon the source audio at that export phase.
This I don't understand. Can you explain further? How exactly do you think an export alters the original source file?
These "lossy" things range from simple stuff like attenuation or amplification, to audio filtering and anything else that Premiere throws at the sources.
PP doesn't do those things without user intervention, meaning you added an effect, so I'm still not clear on what you think is being lost when you export a .wav file.