Massive decrease in egg output, and 24-hour old eggs with blood vessels and beating hearts....

[Allsfairinloveandbugs]

As far as the drastic egg count reduction, the first thing that crossed my mind is snakes. Your temps are cool in the mornings, but plenty warm enough by the afternoon for snakes to find the eggs. I would try gathering at least 2x daily for a short while and see if you get more eggs. Large snakes like the one below eat 4 eggs each and every day, and often there is more than one. Every time I have a drastic drop in egg production, I know a snake(s) has discovered the nest boxes.

 

[erkme73]

As far as the drastic egg count reduction, the first thing that crossed my mind is snakes. Your temps are cool in the mornings, but plenty warm enough by the afternoon for snakes to find the eggs. I would try gathering at least 2x daily for a short while and see if you get more eggs. Large snakes like the one below eat 4 eggs each and every day, and often there is more than one. Every time I have a drastic drop in egg production, I know a snake(s) has discovered the nest boxes.
View attachment 3575098

Yikes. That's blood curdling. I suppose that's possible. I have cameras in the coop, in the nest box, under the coop, in the run, and all around the area outside the run. There's always a hen or two in the boxes so I cannot see under them. I suspect if a snake did manage to get in, the chickens would go nuts?

I'll definitely start to collect AM and PM.

Still dumbfounded over the developed eggs in <24 hours... And the weak yolks...

 

[coach723]

I suspect if a snake did manage to get in, the chickens would go nuts?

I have actually found snakes in the nest box with a hen sitting on it very contentedly. Quite a surprise when you reach under her to check for eggs! Thankfully, that hasn't happened a lot. And the snakes were not as large as the one pictured.
I check for eggs several times a day, and am confident that my egg drop last year was not snake caused, but it's certainly one possible cause in the grand scheme of things.

 

[Allsfairinloveandbugs]

I suspect if a snake did manage to get in, the chickens would go nuts?

When I yank a rat snake out of a nest box and carry it out of the coop and run, hens that are nearby as I walk past will give a growl. So they know it's potentially dangerous. However, two weeks ago I went to gather eggs around 11am. There was a hen facing into the nest box, and my view inside was blocked by her fluffy butt. As we know, hens will often try to make another hen leave that is in their preferred nest box. But this time, I heard a rat snake vibrating its tail as a warning for the hen to stay away. I quickly moved the hen to see a rat snake in the nest box, it's mouth full with an egg. I grabbed the snake by its tail, and as I carried it out of the run, it expelled two runny eggs it had already consumed. And yes, as @coach723 said; snakes will slither right under a hen and eat eggs AND chicks from underneath her. When this takes place in the dark, a broody can't see well enough to defend her chicks or try to escape. There are also instances where snakes have strangled hens trying to defend eggs and chicks, not because the snake can consume the hen, but because the hen was in the way of what the snake wanted. That is the reason I immediately shooed the hen away when I heard the snake give a warning rattle. The hen was very determined to lay an egg in her preferred nest box, even though it was occupied by a snake in the middle of its daily meal.

I too have no clue why you are finding eggs with embryo development when your eggs are gathered daily. I thought that possibly your kids are overlooking eggs when they gather them, but that doesn't make sense if your neighbors are seeing the same. Let us know if you solve your mystery!