Can a moderator/high reputation member "force accept" an answer?
No, they cannot. The reason is that an accepted answer is the answer that helped more the user who asked the question, and only that user is able to report which answer helped him the most.
Who asked the question can only accept the answer that helped them the most, not the answer that is more useful for future readers too. They could accept the answer with more up-votes, but that doesn't always happen, as they could find more useful another answer with less votes.
With a question without an accepted answer, there is something that everybody can do: Up-vote the most correct answers, and down-vote the answers that are completely wrong, or not useful. In this way, future visitors would see on the top the answers that are more useful/correct.
There are some feature requests on Meta Stack Overflow about allowing moderators to accept an answer for those questions where the OP is not anymore an active user, but those requests have been down-voted, or/and rejected.