I'm really not sure why motherhood is relevant to this. One can be an adoptive mother or "mother" someone which is a metaphor referring to nurturing them. It doesn't mean you actually have given birth to them.
Yes, exactly this re motherhood.
Can you define your alternative version of womanhood? Why would a male person be considered a woman, in your view? Rather than a different sort of man?
So, if you're walking down the street and you see a woman, how do you know she is a woman? Do you have a look up her skirt to see if she has the right bits (rhetorical question - of course you don't)? But you can still identify a woman. So for me, this indicates a cultural identity which is more than purely the presence or absence of the right body parts.
Maybe you disagree with this and rail against this sort of identification. But then, how do you know what you do or wear every day, as a woman, (I'm assuming you are one sorry), is not in some way guided by this identity which you may have taken on subconsciously. What if someone without a vagina has, for some reason, done the same? To them, they are a woman or girl and they find being referred to as a man or boy distressing. Same goes for trans men, but the other way round, obviously.
This is my opinion. You may not, and probably do not, agree, of course.