Don't forget... the chemical entering the device can not be calculated from the deposits remaining. (Just because there is less Chlorine, does not translate into it not being from the water. The minerals suggest it is mostly water related, while the carbon/oxygen suggests it is tar/oils.)
Some things just stick better to carbon, and to heat-elements of specific ionic bonds. Other things refuse to stick.
The good news is... Anything stuck to the element, is not in our lungs.
You need to get a similar analysis of raw-fluid before use, listed as percentages of the volume. Compare the used level of liquid, to the deposits that remain, as the same percentages, and you can clearly state... "The heat element may remove up to X.XX% of elements X,Y,Z and will collect and carbonize up to X.XXmg of material in the process of vaporization." (You would have to dissect the whole device, and remove all traces by hand. ultimately, you should dissolve each separate known compound, to determine the true molecular volume, but that is a bit much. Save that for the FDA.)
So much for the whole, "Not breathing in carbon.", argument. (Though, it is obviously a lot less. Also being oxygenated, indicates that it is not completely carbonized.)
Being oxygenated should also indicate that an oxygen-stealing chemical, like peroxide, should assist in the removal of the deposits.
Minerals can not be avoided, as they are part of water, and most water-diluted materials that are not for medical use, or for scientific chemical use.
Where did you get an XPS? Like everyone has one in their garage!
Curiously... what deposits are along the inner walls of the mouth-piece and air-channel?