I’m new to Jolla phone (Batch#3 and still waiting) - therefore there are many questions 
How difficult is it to switch the other half ? I mean would it be feasible to design a car holder TOH where you put your Jolla while driving and on arrival you take out the phone and reattach your “normal” TOH ? The car holder could be designed to charge or maybe some other functionality (e.g. car2x)
It is not difficult, but I still wouldn’t bother to change the TOH everytime I go for a drive.
Better option would be making a custom TOH that is on your phone all/most of the time and has the features you want built in, or easily attachable. Maybe like TOH with wireless charging and modding your car holder to have wireless charger, or TOH with exposed power pins and modify you car holder to have matching pins aligned with the TOH when the phone is in the holder.
If you just need the mounting option, my TOH that is compatible with Magsafe accessories would already do the trick. A TOH that supports Qi charging or something would be thinkable too but I lack the electronics expertise for that.
I agree that you do not want to change the TOH constantly, as that will lead to high wear and tear of the clipping mounts, you certainly do not want to do that every time you get into your car.
Not that I am a J2 user but a ToH that had the geometry for a quadlock (or similar) mount would be neat. Not sure if you guys get these overseas but given this picture is from a LHD car I suppose you do.
Quad Lock - yes please! I use it in my car and on my bike.
