I just wanted to take this opportunity to thank you ROBIN W for suggesting the plugin “widget logic”. I too am having the same sidebar problem as described above, and the Tweaks Plugin simply didn’t work for me. I even tried creating a forum.php in my theme-child, and was able to remove the sidebar by removing the call to the sidebar, but when I made a call to a specific sidebar, it simply wouldn’t work: it always reverted back to the main theme sidebar no matter what I tried.
“widget logic” solved this issue. In the Main Sidebar, I can now add a LOGIC code, such as the ones you posted, and now the widgets either display or don’t dislay on the bbpress forum pages. Case closed!