Thanks for your detailed query.
Here are some points for your understanding.
1. Forum root behaves like archive pages e.g. category, tag etc so the Site Hero will not display.
2. Forum individual topics behave like single pages so the Site Hero will display.
Now, you want a different control so the pages of forum should not display Site Hero but the normal blogs posts should be.
I have just forward it to our developers and i will feedback you after receiving their response.