There's been very few instances where Danny has done something real bad before and that was burning the Tarlys who didn't bend the knee (which is like what everyone would do a part from the burning alive part). Beyond that most of it has been just Danny wanting the Iron Throne, Danny has always said she just wants to take KL, even if there's casualties, again that's pretty much standard for anyone that tries to take a city.
I don't think Tyrion or anyone could see that some bells ringing would make Danny go physco and burn innocent people alive. Before that all she was doing was destroying the armies and battlements.
Like Varys only evidence to show that Danny is unstable is that she was sitting alone at a party and that she was sad because her dragons and friend had died.
If she stopped completely before the slaughter everything would be fine. The fact that her madness is triggered by some fucking bells is completely dumb and no one could have seen that being the tipping point.