I think I'm having a similar problem. I have 4 hens and 3 nesting boxes. The mother hen is still laying in the box every day, but her three daughters (who have laid in the boxes every day for more than a year) haven't laid any eggs for at least a month. I'm wondering if they're laying in the yard somewhere. There's a bush that they like to hide under, but it's too dark and secluded for me to see. I thought they might be laying in there, but I don't know why they'd do that when they've been happily laying in the boxes for so long. They've been fighting a bit though, and I think they may also be moulting, so maybe they're just not laying at all.
On your other problem, I haven't tried oyster shells, but I use she'll grit. It's $4 a bag (several kilos) and lasts for at least a month. I forgot to add it for a month and got the soft eggs. I started adding it again and the eggs went back to normal straight away. I hope that helps.