The wedding is Black Tie attire.
Yes, renting a car is essential as taxis are extremely hard to come by on the lake. If you happen to come across one, they will also charge you an arm and a leg. The roadways are pretty normal to navigate with us NY/NJ/LA drivers.
You won't be able to spend USD in Lake Como, however they do take Credit/Debit Cards which will convert for you. It may be cheaper to convert here in the US versus converting in Italy. Try to get Euros beforehand to have in case.
Yes, we must always have our passports on us, due to Italy law. The cops may stop us at any time if they to check for our identification and the only legal identification is our physical passport. Photocopies/Photos/ Driver's License will not count. If we do not have it we can be fined or taken into custody.