Yeah, I think that's when engineers figured out that maybe airplanes shouldn't have right-angle openings and stress points, but rather round/oval ones like the ones designed for ships and subs.
Yes. They didn't know what was going on until they were able to recover a Comet that had crashed and found the stress fractures radiating from the corners of the windows. Prior to that recovery they had lost a few Comets but were never able to find them. I think they were all lost over water but I might be wrong.