Wow the Cirrus is looking pretty good for how much damage it did to the Metro. Flight surfaces are still there and it's only broken in half of the parachute landing. Guessing it might not have been controllable after the collision.
> The plane flying missing half the fuselage suggests to me that fuselage was excessively strong (and therefore heavy and expensive).
It's probably the opposite: that the fuselage was only strong enough to resist the air pressure differential, and that the real structural component was the cabin floor. And that, luckily, the control cables for the tail were routed through the cabin floor, instead of through the top of the cabin.
It's not the cabin floor. it's what the cabin floor is attached to. Most fuselages have all of their longitudinal strength in their stringers, which are usually run along the floor. If the Cirrus had ripped through the bottom of that plane and not the top then that jet would have torn apart.
Stringers in a plane seem akin to stringers in a car frame... Cars have moved away to unibody designs (where the shell of the car is also the structure) because it is stronger, lighter and cheaper. Unibody designs have been the norm for almost 100 years now.
The structure would be designed around an adequate fatigue life. I wouldn't be too surprised if fatigue considerations were such that in "normal operations", stresses could well be 10x lower than the ultimate material strength.