As a wedding attendee, you do not need a visa to travel to Scotland. Fun fact: The bride and groom are having to apply for a marriage visa.
Yes, you will need a passport. For more information, including passport requirements, please visit: https://travel.state.gov/content/travel/en/passports.html
Inverness offers a wide variety of lodging options, including Bed & Breakfasts, vacation home rentals, or even a Best Western. Websites such as Expedia and Home Away can show available options, in the city, and in the surrounding countryside. Urquhart Castle is located just 15 miles from Inverness, so find the lodgings that best suite you!
No, kilts will not be mandatory for the wedding. However, if you'd like to rent a kilt for the occasion, there are several shops in Inverness that can offer a rental (just like a tux) for the event. Please contact the bride for information of preferred tartans.