Third, a definition of life beginnings would need to include all life, not just humans. So "life begins at conception" is not a scientific belief, but a theocratic one.

Exactly. The sperm and egg came from living bodies that created them. Life begins from life. Life doesn't not begin at conception, it continues at conception. If life "beings' at conception, then that must be there was no life before it. So were the father/mother not alive?

It's an argument via "I like my definition", definitional arguments are not science, they are moral arguments. There are good reasons to be prolife, but this is not one of them.