Some pertinent points...
List D versus List C: The main difference between the two is that List C contains all EU, UK, Schengen, and respective territories; those Countries that they have existing entry agreements with as part of a larger affiliation. The US will never make List C, just as no other countries, except The Vatican and San Marino, will make List A. List B appears to be reserved for EU/Schengen/UK countries with very low risk...but no country has ever been on the list, so hard to tell.
Vaccination: At this time, Italy could care less if you are vaccinated for entry purposes.
The requirements apply mostly to where you are coming from for the most part, there are provisions that extend back 14 days from entry to define "where you came from"
So in your case, if you go to a List C Country, meet all their Covid entry requirements (testing, quarantine, vaccination) stay for at least 14 days (I would not try to cut it close, make it 15 or 16), then Yes, you may enter Italy, however you still need a negative test within 2 days prior to entering Italy. To a degree, not any different than just taking a Covid flight directly to Italy.
Going through another country and quarantining there seems to be the only way to get into Italy for US residents
No, this is incorrect. At present, you can fly on a direct designated flight from the US to Italy, and if you meet the test protocol (One prior, Rapid test at departure, Test on Arrival), there is no need to Quarantine.