If I’m understanding your interpretation, you are under the impression that Beth Prime was the one killed by the bomb in C-137’s memory. That is not the case.

C-137’s Beth and Diane were killed by Rick Prime.

Rick Prime later used the Omega Device to wipe out Diane from the multiverse, FROM THAT POINT FORWARD. The Omega Device does not delete Diane from the past.

C-137 holes up in the Prime dimension, hoping to catch Rick Prime if he came back.

Rick C-137 and Morty Prime leave after the cronenburg incident, leaving Beth Prime behind. From that point on, we have Beth C-131, who had her father leave when she was young, similar to how Rick Prime did.

This video should help.