1. What are the essentials in a packing list for a crewed charter?

Conventional wisdom is to bring half the clothes and twice the money that you think you need.
If you have a favorite sun-screen you might bring it. Limited selection.

2. How much input will we get on the itinerary/destinations?

Have a list of places you'd like to go. Don't make any a "must do." Sailors have intentions, not itineraries. The wind and waves decide.

3. How much should we plan to tip our crew at the end of the charter?

10-20%

4. A few of us are certified divers but I'm not clear on whether we will be able to rent tanks and dive equipment and store on the boat or if it's too much trouble to mess with.

Do a rendezvous dive. Dive company will pick the divers up at your boat and rent them equipment.

5. Any other pearls of wisdom for do's and don'ts for the trip?

No long pants (IMHO). No hair dryers. Plan a stop at a dock about half way through the trip... you'll appreciate it more.