I see the same code. It continues to be perceived with a difficulty.
You see, it's not the feature for miniBB. You are trying to implement what was already suggested to us 7 or 9 years ago. Such forums structure is bad from the usability point of view. When user is lost in tons of information, he loses the focus, loses the interest. The current miniBB's structure, with forum titles and super-titles, is fully enough for 99% of forums. That satisfies the concept.
These modifications are up to you anyway... thanks for sharing!