I am seeing this another way.
1) Get a loaner that you are happy with.
2) From there give no push for your car to be completed.
3) Continue to drive loaner.
You are making a monthly payment and the more of these payments you make, the more of the Fiat 500L you will own. Meanwhile, you have no maint cost associated with driving (driving loaner, not 500L) and you are keeping your depreciation lower since you are not adding miles to your car.
Most importantly, it seems to me that every mile you drive the 500L puts you closer to risk of another long delay type repair. Not driving the 500L means less likely to encounter another repair.
I would drive the loaner as long as possible and see if the dealership forgets that your car even exists.
When you do get that 500L back, I would recommend unloading it, quickly. Word seems to be getting out that these cars have a parts supply issue. That issue is likely to only get worse as the cars age.
At least, do not keep the car longer than the warranty. Who knows how many different loaner cars you could have between now and the end of warranty.
