1.A lot of companies will rent a car overnight. Do your research. All the big companies have locations on every island. Yes, be concerned about getting back late. You may need to hop a flight to another island, though this is cheap, but staying overnight in a hotel or resort is not.
2. I don't know what the Luau Excursion is. There are many luaus on the islands though. Maui has the largest selection and best luaus
3. You don't need a 4WD on any of the islands. If you plan on going back country and off of the main roads, you will, if things are wet. Kauai is always wet and so is Hana and Hilo on Maui and the big Island. Don't worry about getting 4WD. In an average day, you won't be offroading.
Don't get ripped off!! You may be able to get better rates if you stay with the same rental company the whole time and book all at once. Check out these Hawaii Car rental tips
Have a great time!