Regarding marking topics as solved: I now changed the settings in the following way:
- you can edit your posts as long as nobody replied
- you can rename your post forever (the function is in the dropdown menu behind the reply button) - you can use that to add [solved] to the topic
- you can change the tags of your post forever (the function is also in the reply dropdown) - you can use this to add the solved tag to the topic
Since the solved tag is a "secondary" tag in Flarum it can not be sorted above the primary tags. Maybe someone (@ach ?) could come up with some clever custom CSS to sort it first in the post's tag list.
Finally, I'm gonna close this post. It's become rather long by now and moving into all kinds of directions. I would prefer having single-topic threads instead. So if there's anything else that needs to be addressed in the forum software, please open a new discussion with the forum help tag.