Hello,
So, I finally got all clarifications I needed. I'll post this as a future big warning for people in the same situation. I went to a small customs office where I experienced probably the ruthless behaviour towards me that any administration employee has ever gave to me (anyway, off-topic):
No, you can't have form 15.30 managed without the car physically in the customs office because it's an exported good. In other words, form 15.30 is not a temporary authorization to have a non-imported car but more a temporary import. All exported goods need to be there at the time of their management by customs office, it doesn't matter whether it's an old car or a big truck going to China. It doesn't matter if it's a temporary authorization to drive in CH (form 15.30) or anything else. It is considered an exported good and "there's no discussion on that"
No, the certificate of the transaction for the car does not matter, not any official non-Swiss document stating the car officially belongs to someone else living abroad. They don't care, it's Swiss customs, it's your problem to show them the car, they won't bother by looking anything else than your car.
No, there's no other way to do this. You don't show the car, you're considered to have the car in CH and, after 2 years, to not have fulfilled your duties by definitively importing the car in CH or getting the car out of the country. Now, coming to the options left for those that find themselves in this situation it's pretty simple:
a) Do whatever you need to do to have the car physically in the customs office. Buy the car back, ask the buyer to do you a favor, whatever it takes. This might seem not practical but it is the way it should be done.
b) Go to a bigger customs office where they're really busy with the 200 trucks per hour and 100 imported cars per day they have to manage, lower your karma level by lying them and telling the car is of course out in the parking lot between 2 trucks and cross fingers that they won't decide to check the plates and the serial number on the car. Because as a customs officer told me: the fact that an exported/imported good needs to be in customs at the time of declaration doesn't imply they're going to check every good there. This option is likely illegal but if it works it's obviously more practical than option a. But you take more risks.
c) Don't do anything and wait on your couch for the expiration of your 15.30 and start wondering about the consequences of that, which I don't know. Probably a good fine if not more. This is neither practical nor recommended :-)