I'll pick on your last two sentences, if I may:
Should content that points out errors with the OP's logic/understanding be posted as a comment or as an answer?
The ultimate guideline for these things (as far as I'm concerned):
- If it answers the question, it should be posted as an answer
- Otherwise, if it explicitly or implicitly suggests a change to the question, it should be posted as a comment
- Otherwise, it probably should not be posted at all
Granted, that's still somewhat ambiguous, but all the ambiguity is in the meaning of "answers the question" and "suggests a change to the question".
There are some cases where an OP makes an erroneous assumption and correcting that assumption actually answers their question. In those cases, you'd post an answer, not a comment. For example, if a question takes the form "I want to X but can't because Y; how can I X?" where it turns out that Y isn't true, then posting that Y isn't true and the OP actually can X the way they wanted to is a perfectly legitimate answer.
In other cases, correcting the OP's erroneous assumption doesn't actually answer the question, and in those cases you'd leave a comment. Even just pointing it out is enough - if you bring the OP's attention to an error in their question, that's the sort of thing I consider an "implicit suggestion" to make a change.
If such a comment is valid, then what should be the goal of the comment?
I think the goal is to get the OP to rewrite the question in a way that doesn't include their erroneous assumptions. That falls under suggesting a change to the question, so it's a legitimate comment.