The 10X rule doesn't apply to physiological dosages/levels. Otherwise a TRT dose of 150 mg QW would result in a TT of 1500 and that doesn't happen - or happens only rarely. But there is lots of evidence showing the 10X rule applies when using supraphysiological doses.
Contamination doesn't have to be bacteria. It could be from heavy metals, other chemicals and maybe even from whatever the raws were cut with.
Regarding your first point, that is not my experience. I've seen test levels rise in a linear, dose dependent manner. I'm not sure what the biological mechanism would be that would allow test to increase in a greater amount the more that's administered although I'm always eager to learn. Again, years of cycles (UGL and script), years of bloods, years of TRT has not led me to believe that it's true in my case.
To your second point, I fully agree. Anything that's not test, BA/BB, or the carrier oil is a contaminant. It's definitely a concern.