wpForo doesn't need any prefix. wpForo tables have _wpforo_ secondary prefix, it's not a primary prefix. So your forum table prefixes should be anything_wpforo_ . If the staging_wpforo_ is ok. You just need to make sure the board ID is correct.

Open the _wpforo_boards table and make sure the boardid = 0 of the first row. If you have just one forum page, then there should be one board with boardid = 0.

Fix it, then go to wpForo > Overview admin page and click

WPForo wants to use the default table prefix of wpforo_. Can I edit a config somewhere to change where the WPForo application points to change the table prefix, or do I need to create a staging site with its own independent database so it can use the default table prefix? 

I just created this so I can delete this staging instance and recreate it with a separate database if that is preferred. But my site is 17gb and 90,000 posts so it is not as easy as just changing a table prefix somewhere.