We did this tour three years ago.
1. Not really, but know that the tour takes you to some remote places, which can make a bus ride seem long. However, the bus does stop quite often and each trip is broken up nicely.
2. No, because most of the towns are small and easy to see in a few hours. We took time out of sight-seeing to hit the beach every chance we got, therefore not seeing each town fully.
3. No bad hotels. Know that most are small, family run hotels.
4. We arrived a day early and walked around Athens and The Plaka. We also went up Fillopopau Hill at dusk for an awesome view of the Parthenon. After, we flew to Naxos for 3 nights, then took the ferry to Santorini for 2 nights. We then flew back to Athens for a night before heading home.
5. The trip was fantastic! The people are super friendly, the food is great, and the beaches are awesome!
Take a swimsuit and hit the beach every chance you get. Take sandals that can get wet, like Chacos or Tevas. You'll want shoes with a back. You won't need a jacket, only a light sweater. It's the easiest trip to pack for because you'll live in shorts and swimsuits. I learned more about Greek history and mythology from the tour than I ever did in high school or college. Being there, seeing the Parthenon and historic sights, really brings history to life.
edit I forgot to say-there is no way we could have planned, seen, or driven the route the RS tour took us on. It is a very comprehensive tour and well worth taking!