Let's say an objectively wrong answer was highly upvoted, selected as Best Answer, and then all the normal options for recourse (comments, flags) were purged by a mod with a political bias in line with the wrong answer?
Anything that can be done?
Well a mod can't purge flags. They could decline them, but of course an answer being wrong isn't grounds for any flags, so declining them is entirely appropriate. A mod also can't remove your downvote, which is the appropriate course of action for when you think an answer isn't useful.
As for the comments, it's possible that the way in which you went about explaining how the post needed to be improved was not constructive, if so, consider trying to present the information more constructively, such that a mod no longer has grounds to remove it. If you feel that there were no grounds for removing the comment, and that it was important information for readers of the post, then you could consider posting on that site's meta or using the "contact us" form to discuss the appropriateness of the deletion of the comments (but you should be well prepared to explain why those comments really needed to be there and didn't merit deletion for any reason; such requests tend to be viewed rather skeptically, for good reason).
If you know the correct answer to the question, then you could provide your own answer and, in that answer, explain why it is correct and other solutions are not. Note that it's not appropriate to post an answer just to critique another answer; you need to answer the question first and foremost, but as additional information you can explain why other approaches are not acceptable answers. There are a lot more protections for answers over comments; there is a much higher standard required for them to be deleted, and deleted answers can be seen by more users, so if there really is inappropriate behavior going on there are more checks and balances.
As for the acceptance itself, you have no recourse there at all. The question author is free to accept whatever answer they want, no matter how wrong/bad anyone thinks it is.