Just my POV:
a) to your examples: if a comment is genuinely misleading about something you deem important then it would be best to add a clarifying reply for the next readers because a dislike/downvote wouldn't change that someone might take the misinformation for truth.
(e.g. in case of Dislike/Downvote feature: some might even think you'd just be downvoting a comment to hide an actor's scandal or something, so it's always best to truthfully correct misinformation)
If something is genuinely offensive you can report a comment.
'Unhelpful' seems very subjective, we can share our personal POVs about shows and have totally different takes on a show because our tastes & opinions & interpretation are subjective too, so just scroll if you don't want to engage? ^^
b) I feel like MDL has already a very negative atmosphere at times (depending on which show's comment section you're on and depending if it is currently airing etc), not everything about MDL is neg vibes luckily, but it can be downright nasty at times.
So personally I don't see this feature with outweighing pros vs cons, because while you want to restrict mass-downvoting you also have to realize that some will use an easy feature like 'anonymous downvote' to e.g. downvote comments praising actors they don't like and haters of a show can also downvote positive comments etc...
c) Ia with commenter Kokuto, if you rlly want to 'surface' pos comments by moving comments then it would destroy the chronological experience esp when a show airs ^^ Personally I don't need to see the most voted comments on top, I want to see what others have to say about each epi and hey if they have something neg to say it can still be interesting as long as they don't get insulting and explain their POV.