score:6
It's been a long time since I did Havasu falls but I can at least answer #3--no guides needed. The trail is obvious. Note that there is a 2,500' elevation change on the hike, this isn't a walk in the park!
Beware that considerable advance booking is required as there are limited accommodations. Doing it as a day trip isn't viable for most people.
The main airport is Phoenix, this is some hours away. Flagstaff is a couple hours closer, Kingman is also in the area. I can't address the economics of the options, it was always a road trip from Phoenix for me.