Good thing you found that! And your replacement caliper! Could you imagine coming down
Valkyrie hill and that failing!
I've seen these calipers apart and the metal is solid throught that area. With that and the cross bolts running throught it, it is most likely fine, although its probably created a stress riser. I wouldn't blame you for tossing it either.
I agree that the backs are under utilized. With the weight shift these cars see under heavy braking I can understand they reduced the bias or we'ed be locking up the rears all the time. None the less once you increase the spring rate some of that weight transfer goes away. Wish it was adjustable.