That was a peculiar story!
But young moms, eating up the kittens, isnt that unique. If they are stressed enough it is even not at all unusual.
I have read somewhere, out in the open, in ferale cat colonies, it may be up to 30%...
They often warn against toms as a threat. The sad truth is, a highly stressed mom may be an even greater danger to her kittens.
Seek once again around. Especielly in all dark corners and places where you human have hardly a possibility to reach.
If you dont find them, and she is no longer pregnant, so they are almost surely eaten up. By her. If some other cat or dog took them, she would mourn and seek them.
You felt the kittens moving, and the vet confirmed. Otherwise, I have heard of very late false pregnancies too. So this owner of a young stud was happy the first wife get pregnant, and they waited. The day for delivery comes, the phone rrrings. The owner of the female! What news!?? They are asking for a new mating, it was a false pregnancy... LOL.
Good luck!