I'm glad to hear it. Your chickens will end up much better off.
As @Brooks_ said, apparently she doesn't know any better. Layer feed is for active layers as they need a lot more calcium to create those egg shells. They even generally say on the bag that it should not be used before some...
OK. A couple of points here.
Chickens don't all start laying at the same time, even if they are the same breed. It depends on many factors including just individual physical variation.
I *really* recommend you stop the layer feed for multiple reasons.
#1- Most layer feeds are lower in...
No, it keeps for a good while. I'd just keep the bag sealed to keep moisture out.
Please note that @SueT mentioned "on the side". That is very important. Your chickens have very different calcium needs (layers vs non-layers). The layers will be much more interested in consuming the oyster...