53 million shares to solve could be chalked up to really really really bad luck. Once in a few years or so kind of bad luck.
No, about 1.2 times in every hundred rounds.
90 million shares means something is broken.
Nope. Pools have had worse than 7.5 x D. I remember when DeepBit had one at 8 x D, and only a little while later Slush had one about the same. A long round doesn't mean anything is broken.