The net result of adding offboard gas, be it O2 or Nitrox, is to fool the unit into thinking you have not created as much CO2. The unit assumes that the O2 in the onboard tank is what you have available for metabolism. As you metabolize that O2 you create measurable amount of CO2 and therefore you have a calculable duration for your scrubber. If you bypass the onboard cylinder you are bypassing the pressure monitoring that informs the computer as to the amount of O2 that is used in the calculation. These are the volume calculations that yield the CO2 for fixation. There are other factors in the scrubber duration calculations and eventually they will determine the "filter" time that remains and is displayed on the status screen. The fact that you are deceiving the computer with this method forces the computer to yield somewhat inaccurate information regarding the usage parameters set by the manufacturer. That is your choice, but you damn well better know what the hell you are doing if you want to avoid injury. At this level of off-standard usage, the optional CO2 sensor is an ABSOLUTE necessity. If you are going to push the absorbent farther than you were instructed, you must have the CO2 sensor installed. Doing any of this is at YOUR OWN RISK.
As far as the actual content of the loop, the O2 sensors do track PO2 and therefore provide data regarding decompression/NDL limits accurately as long as the only two gases are O2 and N2.
There are a few of us that have pushed the limits of the Explorer, but we understand that what we are doing is for our knowledge and understanding. If you wanted a full trimix/100m eCCR and bought an Explorer, you made a mistake. That being said, I have both, I instruct for both and love to tinker. The Explorer is a fantastic unit when used within the manufactures specifications