I'll help you out with what I can based on my stay 2 years ago.
1. Was a Gold at the time, received an upgrade to a partial ocean-view room without requesting anything.
2. I think Roberts runs a bus shuttle from airport to Waikiki. Works kind of like Super Shuttle
3. There is a TON of stuff within walking distance. We had a car but rarely used it. Restaurants, shopping, etc.
4. I think there's a Hertz office in the Marriott but I may be wrong.
Enjoy - we had a good time here and would go back if we didn't like Maui so much!