Re rear seatbelt bolts
The bolts will come out, but the problem is the end of the thread is exposed to the elements underneath the car, and therefore rusted in. Very unusual for a Fiat lol!! If you can get at the bolts from underneath the car, spray some WD40 on them and start undoing them. As soon as they start to come out, spray some more from inside the car and do them back up again! This sounds odd but its the best chance of not shearing them off. Once they're out smear some copper grease on the threads before putting them back in. If this fails then you can use a nut and bolt, preferably stainless bolts with nyloc nuts.