Answering only one of your questions: "Should one get the insurance while driving in Sicily?"
I have no stats on this, but I'll bet Sicily is a place where there's a greater chance you might want CDW (damage to the rental car); I mean a greater chance than driving in the US.
I think car theft insurance is mandatory in Italy; and some form of CDW coverage, if not required, is probably a really good idea. But it isn't cheap, that's the problem.
In countries other than Italy (and probably Ireland), you have a couple of other choices, other than paying for the rental company's CDW: redit card coverage and Travel Guard. But not in Italy; and of course there are reasons why they don't cover Italy.
And lastly, an IDP is, technically, required by Italian law. Most of the rental companies don't ask to see it, so some people figure they don't need it (I rented before in Italy and the rental company didn't ask to see an IDP) but that's incorrect as to Italy. We've been told that people who have had accidents in Italy, and didn't have an IDP, wished they had gone to AAA and paid the $20.