Regarding the cracked crank, that is, if you can stand one more opinion...
I noticed that there is a black line on the back of the spider, above the crack. Looks like someone noticed it and marked it at one point. If so, the line/crack appears to lead to the spindle. Maybe the spindle or mating taper in the crank are bunged up (to use a technical term), and someone overtightened it to keep it from coming loose?
Also, there is another possibility I haven't heard in all the talk of failing parts, not just crank arms. Parts sometimes break because of flaws in manufacturing. It happens. That's why high-reliability hardware, whether mechanical or electronic, undergoes special manufacturing and testing.