Each itinerary allows you to see as much as possible without too much driving. Most days are under 3 hours of drive time but could be up to 4 hours a day depending on your accommodation location. Driving distances are located in the description.
Flights into Iceland land at different times of the day depending on where you are traveling from. The arrival and departure days of the itinerary are flexible and allow for plenty of travel time to and from the airport.
That depends on the size of the group and your own preferences but all of the sights in this itinerary can be reached with a regular 2wd (however I always recommend a 4wd for winter travel).
You can do it either way.