Oddly enough, people confess to awful things out of the blue in all kinds of situations. I don't think training of any kind would have made a difference; I think he wanted to let it all out. Sure, any situation can turn dangerous - I've been on a bus where someone said something harmless that got another passenger totally wound up and he started walking the aisle saying he was going to beat us all to a pulp. And priests hear confessions all the time and they haven't been trained to "deal with" this kind of thing. My father heard all sorts from his parishioners - and he was Anglican and didn't do confessions as such. There were quite a few wife-beaters in our community, and they all talked to him. Not one ever harmed him, though one tried once after he had been to jail - and one of the others stepped in to defend him ! ("NOBODY hits our vicar !")
I think it's important to remember that ANY situation can turn nasty, and to act sensibly with anything that comes up. I think this was simply a tricky situation beautifully handled. If the sitter had "freaked out" it might indeed have turned nasty, but she kept her head - without any special training - we can all do that if we make the effort.
As to unanswered questions - I can't see any, myself. He confessed; he clearly did it;, the reader seems unfazed - where's the question ?