Skeletal DNA is part of the puzzle but would not be enough by itself. Replicants are assembled, not grown whole. Points backing the "Frankenstein's monster" style of construction include Chew saying "I just do eyes!" when confronted by Batty in Blade Runner, and it being stated that J.F. Sebastian's specialty was replicant hands.
The DNA of a replicant would be similar to a biological chimera. Because any one part could have different code, having only parts of a skeleton isn't sufficient to reverse-engineer the full spectrum of capabilities of the original machine. The DNA in the cervical tissue or other reproductive organs would be closest to the offspring. Wallace didn't have this.
Because the naturally born replicant will itself be
a hybrid of Deckard and Rachael,
they might run into the same problem of course. The child's DNA won't completely match
Rachael's, but it's assumed that obtaining Deckard and the child
will help them complete the puzzle.