Answer to question #1: No, the driver side air bag is always active no matter what. There is no weight sensor for the driver seat. It is assumed that who ever is driving is old enough and large enough to safely handle driving a car and therefore able to be protected by an airbag. (There may be some exceptions to this rule for little people).
Answer to question #2: No, you can not disable the sensor in the passenger seat. There is no switch or turn key that makes it so the airbag is always active. That's the short answer. It used to be in some vehicles like pickup trucks you could turn the passenger airbag on and off by inserting the key into a slot and turning. This is not the case in the CX-5, it's now a switch activated by body weight.
The long answer to question number is.... Maybe. It is highly recommended that you do not tamper with any of the safety systems such as airbags on your vehicle unless you are a trained and qualified technician. With any electrical system, such as, SRS (safety restraint systems) there is way to bypass any given sensor. It is possible, it's just not known at this time how to bypass any given part of the system. It could be as simple as unplugging a sensor, it could be as complicated as having to rewire have the system to bypass one small sensor.
Anything is possible. The reason so many people weren't given you the answers to your questions was because the information you sought, they felt, was not needed to solve the problem you stated. The general consensus is that you do not need to worry about disabling a SRS sensor if you are able to adjust the height of both the drivers and the passenger seat to a height that is acceptable for your girlfriend so she no longer need a pillow. Does that make sense? You came onto a car forum where most, not all, but most of the active members are car enthusiasts who generally want to help you out. They are trying to tell you not to make things more difficult then they need to be. No has seen an in depth wiring diagram for the 2016's and even if someone did have that information it is highly advised not to use to alter the SRS system.
If your girlfriend is still not at a comfortable height (she must have a very very short torso) then I would suggest trying a cushion that is made of denser foam to better transfer the weight through to the seat so the sensor is activated. It may be that the current cushion is just to soft and to cushiony, if that makes sense. Also if both seats are raised to their highest levels and a cushion is still needed, a much thinner cushion could be used.
