Hari there's no solution that isn't disruptive in some way.
Some Flarum features rely on the discussion.firstPost relationship, for example discussion excerpts or email notifications. firstPost is automatically updated when the discussion meta is computed, so by changing post dates, a new post can become the new firstPost.
However for graphical features, many extensions explicitly look for a post with number = 1. So when that post is gone, it's gone. The only way to set a new post with number 1 is to manually edit the database. This will cause the permalink of that post to change, so any old permalink to post number 2 for example will be broken.
At the moment I know very few extensions that deal with post number changes. It's the split and merge extensions from FoF. And they do occasionally cause issues related to permalinks and/or notifications due to posts changing numbers.
In the specific case of best answer I suppose the extension just allows any post number higher than 1 to be selected as an answer.
I'm not sure what exactly you are having trouble with? Do you need to set a post as a best answer? You mean it doesn't render anywhere, or you mean you have a post showing itself as best answer?
