Unfortunately the hard statistical evidence on this is hard to find. I'm not sure why. You quoted many of the types of stats I'd use too. In regard to cause for termination, spend some time with state department of education attorneys (as I've done). They'll tell you what the main categories are... =)
Sorry. Wish I had some really hard numbers for you. The examples you cite are egregious but I'm pretty certain those are outliers, not the mainstream.
Actually, if you look at the statistics, no teachers ever get fired. And those very few that do don't get dismissed for 'academic freedom' issues. It's nearly always insubordination, crime of moral turpitude, or incompetence.
I'm just sayin'...