The addling (mixed yolk and white) might just be caused by the yolk breaking, which seems to happen more easily when infertile eggs have been incubated for a while.It's the addling rather than the infertility I seek answers for btw!
I have cracked open infertile eggs, that spent a week or more in the incubator. The yolks broke very easily, as compared with fresh eggs. I assume the yolks would break even more easily if the eggs spent longer under a hen (21 days, rather than the 7-10 days mine did.)