I just posted a comment to Interfacing microcontroller and mains via a relay. The accepted answer makes inaccurate (and imo possibly dangerous) claims. Should/Could I do anything more than insert a comment to that affect on the accepted answer and down-vote it?
No, you acted accordingly.
There's no claim that an an excepted answer is inherently "correct." It is simply the answer that the original author thought was most helpful to them. If an information in any post is incorrect, correct it. That's why there is an edit function on this system. If you do not have the reputation to edit, you can comment (to request the changes) and down-vote. If the information is dangerous (as in it should never have been published), you can also flag it for moderator attention.