Comments should only have temporary, non lasting, value and should aimed at improving the post by pointing out potential issues, mistakes, and/or ask for clarification.
If a comment is in bad shape or unclear, making them hard to understand I don't see a problem with one person @reply-ing them, asking what they mean with their comment. You can make this as light or heavy as you like:
- Did you mean to post that comment on this post, it seems unrelated?
- It might be me but I can't make head or tails out of your comment, can you elaborate?
Keep in mind that if you leave a comment and @-reply the commentor, you need to be willing to engage in an conversation to work out what is bothersome about that comment, working towards an improvement of the post. If you don't care about the post being improved, then don't comment. Someone else who does understand what is meant will fix the post or engage in a comment exchange.
Don't forget to clean-up your own comments once the issue is settled.
What should be done with comments posted by other users if they have some value but are badly written?
Make sure the value is captured in the post and then flag the comment as no longer needed (I often use a custom reason to explain that the comment lead to an edit and can be removed for that reason)
Should there be possibility to edit them?
Not for normal users, no. Let's not spend another bunch of resources in nitpicking over comments. Moderators can edit comments and I sometimes suggest that in a customflag, for example if link went stale.
Should I flag them for moderator attention?
Comments that don't or no longer help in improving a post or to reach consensus about what to do with a post should be flagged as no longer needed. If all comments are stale, flag the post for moderator attention and ask to purge all comments (on Stack Overflow I was told by a mod the threshold is 4 comments or more). Rude or abusive comments should be flagged without hesitation.
Should there be review system similar to the current one for questions and answers?
No, ideally all comments should be gone once a post reached the quality a site strives for. And comments should have no lasting value and as such they don't need curating.