I'm Italian and just want to give a few replies to your questions (sorry my English).
-"Why there are any people in the background?" The emptiness of places isn't just for a stylistic purpose, neither because of a low budget, but has realistic reasons: the show is set in a stereotypical group of little towns. Now there aren't jobs in country-lands and a lot, A LOT of people moved on in bigger cities or even in the north of the country or abroad. So the south of Italy is full of empty little towns, with spectacular views but just a little count of old people living in it and a lower one of the young. Often Montalbano moves between them and it's almost normal that there are few people and cars. A little bit is the director's choice, but it's more realistic than you think.
-Catarella is "too stupid for police"? He isn't stupid, he clearly has a sort of mental disease/neurodiversity. He has difficult in little everyday acts, but he also has skills in his job, so he can be a productive member of society. It isn't too unlikely. In Italy the state has to help disabled people who have difficulty finding work, paying them a little pension. So if possible public institutions prefer to hire them than pay them to do nothing. By low, all the public offices have to prefer disabled people for some jobs, if they are however able to do it.
-Italian men really behave and gesture that way? Yes, absolutely. I'm Italian and I ever notice any non-ordinary expression in Zingaretti interpretation and body language. He seems to me just strong and old fashioned manly, but not exaggeratedly expressive at all, so, yes, hand gestures are quite realistic. Infact, it's an Italian production, with Italian actors, it would make no sense to put a foreign stereotype about us for purpose. The stereotypes that you can find are all inner, as the old religious ladies and so on.
Now, my opinion? I like Montalbano and especially the work with regional language and the locations. Sometimes I think there is a little to much of machismo in it, but It's a traditional tv show. I really appreciate the mix of investigation, feelings an irony: I love detective story with a funny side, and I valuate more characterizations than action scenes, so it's my kind of show.
