1) Suggestions. . . you can try a 3rd-party excursion company. My travel company (no, I'm not soliciting business
) recommends a tour company called "Shore Trips". You can check their website to compare what they offer to the prices for excursions that you find on the DCL site. I've personally used them for an excursion in Hawaii and their service was excellent. In my opinion, their prices are usually very similar to what you find onboard ship -- some a little more expensive, some a little cheaper. One thing that you'll find with them is that they offer a lot of private excursions, which do tend to be expensive, but you're not on a bus with 100 other people.
2) The two main perks for being on a DCL excursion are (a) they are supposedly scouted by DCL people before they get "approved" and (b) the ship promises not to leave before your excursion returns. It is possible that if your 3rd-party tour is late returning to port, the ship might depart without you. I'm sure it's happened, but the reputable tour companies know exactly when the ship is scheduled to depart. Additionally, Shore Trips guarantees your return on time. If they're late and the ship leaves, I believe they will provide your transportation to the ship's next port.
3) I have never done the "Best of. . ." excursion package. They always seem so big. Sorry I can't be more help on that one.