Moderators cannot accept or unaccept answers on behalf of other users, no matter if that other user is registered or unregistered, a "newbie" or a "veteran".
There are many reasons for a question to end up with perfectly good non-accepted answers. The opposite is also true, with questions sometimes having accepted answers that are not the most up-voted answer, or accepted or highly up-voted answers that have proven to be less than ideal or perpetuate bad habits (in some cases, editors may insert disclaimers into accepted or highly-upvoted "bad" answers, and in extreme cases, a moderator may delete an accepted answer, but this has only happened one or two times in the last few years).
The votes attached to a question or an answer, including the accept vote on an answer, measure to what degree other people have found the text interesting, useful, or "good". In some cases, it may also measure other things, like how witty the text is, how well-written it is, or just the fact that some particular person wrote it.
In the end, readers of the site will have to apply their own judgement and make use of the information that is most relevant to their own situation. An accept vote on an answer only means that the user that provided the question found that particular answer the "best" at that moment in time, according to whatever private metric they decided to apply.
To summarise: Voting is private, and a moderator can not vote, including accepting an answer, on behalf of anybody else. The reason for voting up or down, or for accepting or not accepting an answer, is left to the discretion of the individual user.