Answered: How can I add, let's say 128 characters, of the best answer (when there is one) under the questions?

If you modify the DB directly, you will not fire all the needed events needed to perform the expected action. That can either make the action fail, or make any other Q2A core or plugin feature fail. In other words, even if you don't detect you've broken something immediately, you might detect it at a later stage.

Conclusion: unless you know 100% of the Q2A core and plugin code that gets executed when performing a specific action, don't try to replicate it by modifying the database directly.

Having said so, there's a plugin that takes care of some (if not all) of these issues: https://github.com/amiyasahu/q2a-delete-hidden-posts