Seen that video before and didn’t really fill me with confidence I don’t know enough about crash testing to make an informed opinion. Looks like it deforms a lot to me.
It doesn’t deform, it flexes and goes right back to it’s original shape in the test procedure video.
Materials can deform elastically AND plastically, as well as a host of other more complex means. I would be surprised if it returned to it’s original profile after ”impact testing”.
As for sizing, I bought a Met L/XL and it’s been good for my bucket shaped head. I rode it during the last autumn, winter and spring (temperature ranged from -15 to +15*C). Ventilation seemed fine, although i was very sweaty. I used the thinner chin pads throughout and the fit was comfortable.