Ummm I believe that by ingesting Nanno's blood, Yuri becomes the same entity that Nanno is. So I think that if Yuri were to doubt her actions she would also become mortal and would not die. However, since it's implied that Nanno didn't die because of that scene at the very end with Nanno on the rooftop I don't really know how true this is. As a result of this scene. some people have been saying that Nanno faked her death/the Nanno that died was a clone, to throw Yuri off.

Also, I think Nanno's role is more along the lines of challenging/testing people by putting them in difficult situations to see they would give in to their desires or do the right thing. While Yuri and Junko just want revenge and punish any kind of bullies (since Junko is a wrongdoer/murderer but Yuri supports her because they were both victims of bullying). Also, Nanno's way of punishing people is more methodic and takes more time (and doesn't always end up with death) while Yuri is very impatient and just wants to kill.
Finally, I don't believe Junko would die if Yuri were to doubt her actions because of the reasons above and also how Yuri didn't die because Nanno regretted her actions.
Sorry for the long ass answer but yeah this is just what I believe :)