How do you know? Have you read a textbook on Vulcan genetics lately? We have no idea how likely it would be for Amanda to conceive a child with a Vulcan. Our real-world 21st century science can't speak to that in the slightest.
Err, yes, it can. It should be obvious that a life form that evolved on an entirely separate planet would have even less relationship to us than another organism that came from the same evolutionary heritage as ourselves.
It's a common fallacy that science is incapable of saying anything about things we haven't directly observed. That's a fundamental misunderstanding of what science
is. The whole point of scientific theory is to extrapolate the underlying rules beneath our observations so that we can predict the outcomes of situations we haven't encountered yet. If you can demonstrate, say, that gravity works consistently on the Earth, the Moon, Mars, and everywhere else you measure it directly, and that the motions of stars and planets follow the same laws of gravity that your direct measurements showed, then you can state with great confidence that gravity will behave the same way on a planet you've never seen or visited -- because you understand the universal rule that lets you see beyond the limits of mere observation.
To offer a more familiar example, if you know that driving into a wall at 30 MPH will wreck
your car in your city, then you can be reliably sure that if someone in a distant country you've never visited crashes their car into a wall at 30 MPH, it'll have the same effect. Even in everyday life, it is possible to make predictions beyond directly observed events by understanding the underlying ground rules.
In this case, the ground rule is that procreation is only possible between organisms that belong to the same species or to very closely related species. We don't need to know about life on other planets to know that's true about
us. We know for certain that the only things we could procreate with would be fellow members of genus
Homo. And that's a genus that evolved on this planet. It won't have evolved on another planet. There'd be no direct relationship there, not even a remotely close one -- unless some alien race like the Preservers had transplanted actual hominins evolved on Earth to that other planet.
Anyway, I don't know why you feel the need to object in scientific terms, however misinformed. We're talking about a work of fiction where alien characters symbolize human attributes. Interspecies breeding is a fictional conceit and a metaphor, nothing more. There's no reason to argue that it's something that could actually happen, because the shows that depict it are not making any such claim.
