Use conventional masks? Cant be that difficult, if it's really just an interview type of thing. in fact the lack of motion and discernible detail patterns is probably what makes it not work with RB.
Seven minutes is a very long shot to try and rotobrush. The tool is not well suited to long shots. Rotobrush is not a tool that will just lock on to a shape and track it for several minutes. Rotobrush eats up system resources like crazy and bloats the AEP file. If it completely stops working and the shot has not changed that much then you have probably run out of resources.
I'd cut the clip up into the sections that you will use in the final edit and handle each cut separately. This may be a good project to roto with Mocha. It all depends on the shot.