At Admin Panel -> Settings -> Switches you have an option to enable internal replies on posts. The internal replies switch enables posts within posts -- short responses to the main post, displayed within it at the bottom. You can use this to allow topic owners to respond to complaints, or to allow everyone to respond without making a separate post.

Once you've switched on the internal replies switch, go to Admin Panel -> Members -> Manage Usergroups and you'll find two related usergroup permissions for each group: "Can reply internally in own threads" and "Can reply internally in all threads". Enable just the former to have owners be able to respond and enable the latter to allow everyone to discuss posts internally.

Another possibility to consider is allowing certain types of users to reply internally (perhaps expert fact-checkers or moderators or administrators) while disallowing others. Just set usergroup permissions accordingly.

Configure the maximum length of internal replies at Admin Panel -> Topics -> Post Settings -> "Maximum internal reply length". How long you'd like to allow them to be will depend on the purpose you plan for them.

Note: The permission to post internal replies is contingent on permission to post in the thread. If a user is not allowed to post in a given thread for whatever reason (forum permissions, thread bans, the thread being closed, etc) they won't be allowed to internally reply to any of the posts in that thread either.