Uninteresting research area and contacting potential supervisors
Your undergrad supervisor would be a terrible person if they tried to attack you in the future because you didn't want to work with them for your masters. That's just crazy, but it does happen. You'd be better talking to the supervisor and getting their advice about what you actually want to do. They may know one of the other potential supervisors in the other area and be able to set up and introduction. 

Besides, even if your current supervisor did want to try to harm you later because you didn't want to work with them, wouldn't your rather risk that than get locked into research about a topic you don't like?

You should feel free to contact the more junior member of the potential research group. Do let them know that you've tried to reach the head, that you understand that they're very busy, and that you'd like to work with the group in the future.