"No one will ever be able to prove to everyone else's satisfaction that a foetus...at various different stages of its development...is or is not a living human being and/or a living soul. It cannot be proven in any way that will satisfy everyone."

The problem is, however, that as true as that statement is, we have legally ("legally" used loosely here, because we've never concluded this through legislation -- only through the courts) concluded that we obviously HAVE proven to everyone's satisfaction that it is not a baby because we have determined it is legal to kill them.

We have determined that it is legal to kill them when they are two-celled, and we have determined that it is legal to kill them when they are in the birth canal, seconds from being born. We have determined that we can kill them if we think they are deformed and we have determined that we can kill them if we think they will have red hair.

We have determined that we can kill them if they are inconvenient.

We have determined that we can kill them and make a profit from selling that service.

There is no legal limitation to when we can kill them, even though, as you claim, we cannot prove that it is not a human being.