The desire for clergy not to be mandated reporters goes in the opposite direction from what you suggest. The slippery slope here doesn't lead to breaking freedom of religion, it leads to a religious organization hiding crimes whenever they want.
It is not the opposite direction. It's the same direction in a different system. Their religious system fails if confession isn't only between you and the clergy.
I don't think we want to be in a position where someone confesses that they aided with an illegal abortion, like they're required to by their religion, and is arrested for it. Not all laws are good or just. If mandatory reporting for one crime is made, there's no reason it shouldn't expand to more/all crimes.
Leaving an exception in for the confessional when it comes to mandatory reporting would allow any religious group that had a mandate for secrecy to say, ‘We don’t have to report anything.’”
No, they only don't have to report confessions. They'd still be legally required to report if they discover crimes happening, like other clergy committing crimes. It'd only be things said in the confession box that are safe.
I don't like religion, and I really dislike organized religion, but I also hate giving the state power over people's lives. We bend over backwards to get revenge in our society, to a massive detriment to ourselves. We give up so much just so we can get back at someone else. We need to stop this. Freedom is important. Yes, security is nice too, but how much security does this buy for the amount of freedom it could lose?