egg problems: egg drop syndrome??

Any ideas on my hens' problems below?

They are 2 columbine hens about 9 months old. There are no signs of illness. They are healthy, with bright red combs, bright eyes, normal behaviour, clean vents, normal droppings. They used to lay regularly 1 egg each every day with only occasional gaps. They aren't showing any signs of moulting. They were wormed about a month ago with flubenvet. We are in the UK.

Their symptoms: one hasn't laid an egg for a week. Prior to this, she was laying only every few days for two weeks. Her normally pale blue eggs had faded to almost white whilst the egg production was declining. The other, who normally lays pale beige eggs, is still producing one a day. Her eggs are much smaller than usual, the same colour, normal shape with no wrinkles, but have blobs of calcium deposit on the pointed end. The eggs are normal inside.

We have recently had a three week heatwave in the UK, which coincided with us being on a long holiday for almost 3 weeks, our neighbour feeding the hens, and also just before we left our oldest hen died (sour crop).

They have good quality layers pellets, grit and oyster shell available all the time. I have just a couple of days ago cut out all treats except a small handful of corn in the afternoon.

From research, eds seems possible. However, I can't think how they would have got it, since they are in an enclosed run with chicken house inside and the whole thing has a solid roof as per bird flu requirements. They don't free range in summer after veg garden disasters!!

Do you think it is stress and will settle? But we have been back for 10 days, and weather normal UK temps quite cool for summer since we returned?

And, I can't find anywhere if we should be still eating the eggs. What do you all think? anything I have missed? I wondered about vitamin supplements or cider vinegar?

Many thanks, Mandy.