[drunken monkey]

just so you know, i said 50 degrees c because that is the temp that my hot water boiler is set at and so it is what comes out of my tap.
there is absolutely no scientific reason for that number other than that...

also, sometimes, the place that you need to apply pressure isn't where you think it is.
a warp as you show on your part could start from anywhere between those two opposite corners and the warp itself could be gradual over that entire corner.
what that means is that if your were to try and bend it back innto shape under heat, you would need to do it bit by bit over that area instead of just twisting it over a single line on the plane.