The Doctor's "15 times" line is very ambiguous. I think the most straightforward interpretation is that it takes him 15 attempts to convince Bonnie to change her mind, not that he has had to use the Osgood Box 15 times in the past to suppress Zygon attacks. But either reading is valid.

The parallels with the situation in France do, unfortunately, make the ending seem particularly trite and glib and I'm glad I was able to view the story without those terrible events in mind, because I think it looks tasteless and puerile when placed in close proximity to them.