Castaway is very convenient for the restart of the Dream. Definitely helps ease them back in with a more controlled environment. I've been thinking that them dropping Nassau had more to do with Nassau and their restrictions than Disney's unwillingness to visit a foreign port. Nassau requires a Health Visa for both vaccinated and unvaccinated cruise travelers, and additional testing for unvaccinated... the timing of those required PCR tests potentially don't line up with Disney's '5 days or less before embarkation' requirement, which means that Disney would have to figure out how to accommodate that for those that would require it (at additional cost, I'm sure). For 3 and 4 day sailings, it's much easier to just say "nah, dawg, we don't have to deal with that, so we won't. We're just skipping Nassau for now".
Since Mexico (currently) doesn't require any extra testing or a health visa for cruise travelers, it may be an easier stop. There's always the chance that they could drop a port and add a sea day to try to limit risk on the longer Baja sailings.
For the EBPC, I definitely don't think it'll be sailing the itinerary as it was originally scheduled (although that sailing is still 3 months away, so things could change, but to Aurora's point, Grand Cayman is already off the table). If they can't stop in Cartagena and George Town, that gives them a couple of extra days' wiggle room to find a port that they can stop at to satisfy the Distant Foreign Port checkbox. I'm confident that, unless things get much worse, they can find another place to stop. I'm not too concerned about DCL being able to figure out a way to make the itinerary work legally. Many stops, in many ports is definitely more of a barrier, depending on how safe they feel about it.
I have to admit, I do find these potential barriers to be way more likely to be a problem to the extent that they decide not to send the Wonder over than just lack of crew or the cost of coming through the PC empty. I do appreciate your pointing them out.
Factoring in those things I hadn't considered before (which are all valid points), I'd say maybe my confidence has lowered a tad, so I'm now about 85% sure they're going to send the Wonder over.