I really didn't look at the
DCL excursions that closely because we knew we were going to book private tours for the things we wanted to see, although there were DCL excursions that did the exact same thing that we did on our own because we saw several DCL busses on our tours. I don't know how many are traveling with you so you really need to look at the costs of the private tours vs DCL. We usually have 5 or 6 of us on a tour, sometimes it is less expensive than DCL, sometimes not. If you want to book back up private tours, make sure you check the cancellation policies.
Sorry, I don't know if any of the DCL tours sold out.
I would prioritize what you really want to see and make sure to have back up tours picked, just in case. If excursions do sell out, just keep an eye on them as spots do tend to open up closer to sailing. Some tours are offered for both families and adults only, so if the adult only is sold out, you could possibly get on the family one.
I totally get agonizing over getting tours. I hope this helps a little.