Interesting I came across this thread today. I was shopping for oyster shell yesterday. I checked TSC yesterday. Dumor 5# bag says feed free choice or mix 1# to 20# of layers feed. Mana Pro 50# bag says the same. I also went to Mackeys and they had Purina 5# bag which I bought and I'm looking at, it says, provide free choice next to feed during the laying season or mix 1# to 20# of feed. Not layers feed like the other 2 brands. GCMy comparison (i.e. the reason you shouldn't mix them) is about nutrition, not moisture content, texture, solidity, taste or any other characteristic.
Regarding mixing in oyster shell, its redundant and unnecessary. Layer feed is already meant to meet the calcium needs of a laying female. Offering it on the side is not a bad idea "just in case", but more is not better and if they are not picking around it you could potentially be upping the calcium in their diet to levels that strain their kidneys and do more harm than good.