Yesterday I decided to take a different tact with the WordPress / PHPBB integration. I installed AutoMod into PHPBB and used that to install WP-United. This time, I had no issues. I remembered to deactivate all the plugins first which was probably the cause of my issues in the first place.
Not too much to talk about this time as the install worked exactly as I would have hoped it would. Things seem functional for now. As of now, I have PHPBB running inside WordPress. My next steps will be to figure out how to better integrate things and to try some of the cross posting features. I would also like to investigate using PHPBB as the comment system for WordPress.