Notwithstanding your well thought out and intelligent assumptions of the different operating conditions, if Fiat thought their 500 engines would run better on a 0/30 oil during the winter period, wouldn't they specifically recommend that in their service manual? Again and no disrespect intended, you could be perceived as intimating that you believe you know what is best for an engine you had no part in designing or building. Using Fiats' recommended spec of oil all year round is hardly going to harm the car and of course, there won't be any warranty issues, but, as always, it is your personal decision at the end of the day.
Whilst I agree with what you're saying, if I was to play devil's advocate I'd say that Fiat wouldn't change the grade of oil purely because it would mean that every 500 would go in for 2 oil changes a year and people would see that as an unnecessary expense.