The official SO position is that accepting an answer (or not, or accepting then un-accepting...) is something only the question asker can do. Mods can't do it and developers won't do it.
The idea is that the check mark shows what the OP thought was the best answer, and if the OP chooses wrong, or if things change, then upvotes show what the community thinks is the best answer.
This seems to be the most authoritative question on meta.SO (multiple dups point to it): Moderators accepting answers on user's behalf after a certain time periodModerators accepting answers on user's behalf after a certain time period
And this one: Can a user who is not question owner "un-accept" an answer?Can a user who is not question owner "un-accept" an answer?
Even in the case of a deleted user, Shog9 said "no way, no how:" Allow mod accept for deleted usersAllow mod accept for deleted users