We're all cheap. Well at least I am.
If it were me, I'd refrigerate or freeze the layer feed.
I wouldn't mix half and half. That would give you 2.5% calcium. I'd mix a quarter layer to 3/4 grower.
You can't keep them from eating each other's feed. And, yes, feeding layer feed to 8 week old chicks can kill them.
The best method is to feed grower feed to all the birds and provide oyster shell in a separate container for the layers.
Birds actively laying need extra calcium. Growing birds...