Your credit card will be accepted at 90% of places, so carrying cash isn't a requirement. If you plan to shop at flea markets and farmers markets, carrying cash might get you a better deal.
Early, or whenever you see a good deal. Flights will be available 10-11 months prior to your desired departure date.
You can buy your ferry tickets here: https://villaottone.com/en/traghetti/ To get to Elba, search for ferry tickets from Piombino to Portoferraio. For the return trip, search from Portoferraio to Piombino. Keep in mind, you do not need to purchase your tickets ahead of time, you can always purchase them at the port. If you are not bringing a car on board the ferry, you can purchase a ticket on board the ferry in a pinch!
Plan for warm days, cool nights, and occasional showers. Windy.com is pretty accurate as it gets closer to the wedding weekend!
We suggest one of the following options: purchase a SIM card for your phone at whichever airport you arrive, purchase an e-SIM using https://maya.net/plans/europe or https://www.airalo.com, purchase an international plan through your US carrier, purchase a portable Wi-Fi device in the US and bring it with you during your travels.
If you like them enough, yes.
1 (Italian) hour - so 1:15 - 1:20 depending on the ferry.
Please just watch this video: https://www.youtube.com/shorts/xzdu0zoCaBU?app=desktop
Estimating how long customs will take is difficult, so we suggest going with the flow on this one and purchasing as you go! You don't need to worry about buying tickets ahead of time because train tickets don't sell out, especially in May. You can buy tickets at the train station, on the trenitalia app, or the trenitalia website.