<ol>
<li>Yes, you should flag such contributions.  You shouldn't necessarily flag them as "rude / abusive", unless that's actually the case, but you can still flag.  For example, comments can be flagged as "no longer needed" (the fact that they may never have been relevant does not negate this).  You can also flag and leave a custom comment.  We get that sometimes and the additional context is helpful, at least speaking for myself.  </li>
<li>(I don't think the behavior you describe is kosher, but from a <a href="https://en.wikipedia.org/wiki/Denying_the_antecedent" rel="nofollow noreferrer">logical point of view</a> that doesn't require the consequent to be false, so let me address this.)  <em>You should not "respond in kind"</em>.  That will only lead the situation to degenerate further.  If you think someone's behavior is inappropriate in some way, the best thing to do is flag it and move on.  If you are confident you can address the behavior in a constructive manner that remains in keeping with SE's <a href="https://stats.stackexchange.com/help/be-nice">be nice policy</a>, you may try to do so.  In general, however, this is a high-risk, low-reward strategy.  </li>
<li>A comment downvote button would be a site-wide change requiring the SE developers to get involved.  You could ask there.  That said, (1) I personally doubt it would "encourage better discussion", and (2) I believe it has already been discussed on meta.SE and rejected (see: <a href="https://meta.stackexchange.com/q/3615/187282">Allow downvoting comments</a>, and <a href="https://meta.stackexchange.com/q/140991/187282">Reconsider comment downvoting to discourage rude comments</a>).  </li>
</ol>
