Just to add a bit of information about the forced subcategory selection.
I saw this post when I was looking for a solution:
Minimum category / sub-category dept level to post a topic - #3 by rbrlortie (the solution isn’t necessarily what I wanted, and is outdated)
I registered to @rbrlortie’s forum last year out of curiosity and I saw that when I was in a parent category, the category selector was pre-filled with a default subcategory, which was nice. But I looked at it today, and it doesn’t work like this anymore.