I haven't met anyone with a broken screen due to the force they applied with merely their fingers. Also, the iPhone 6S and 7 (Plus) have TouchID technology, which means that you have to apply a little bit more force on the screen to call up some extra features. It's safe to assume that Apple calculated this and that the screen is quite strong.
As for your last question, I don't know what will happen to the screen under extreme heat.
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