Integrating one bird is usually pretty tough on the single bird...
...but if she's special she might just whip those young pullets into shape.
I guess it's worth a shot if you've got a bunch little egg eaters on your hands.
That stinks!
Was the shell thick and hard?
Were all of them eating it?
Curiosity pecking in not unknown with newly laying pullets,
but yours seem to be going above and beyond!
Adding an older is not a good idea IMO.
It's really hard to say, further observation might give a clue....or not.
I'd keep up with the high protein treats to augment the layer feed(16%?),
at least until they are all laying.
Odd that they won't go into the nests, nest messing has been the number one sign that laying is imminent here...
Might try putting the birds in the nests or on the walking board,
they won't stay but it will pique their curiosity.
Are they confined to the coop, that can also help 'force' them to use the nests.
How long have they been on layer feed?
How old are they, in weeks?
How thick was the shell on the egg you caught them eating/pecking?
Here's some tips on to tell...
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/