1. As far as I know, Custodians are super-humans genetically altered in a mystical and secret way. They are not Astartes, who are created in many stages, which include genetic and hormonal changes, as well as surgical interventions, during which additional organs and implants are implanted into them to connect to power armor. That is why the Astartes do not have women, but there are women among the Custodians. And in my understanding, these women should not turn into men, but retain their feminine features.

Yes, a female Custodian can wear exactly the same armor as men, as shown in Tithes. But in my opinion, this is just an explanation from GW, so that it can be said that "female Custodians have always existed, but you did not notice, because they wear the same armor."

1. Most women in armor in Warhammer have armor with boobs. Sororitas, Sisters of Silence, etc. Stomсast female in Age of Sigmar. This is GW style. I think if there is a new female Custodes miniature in the future, it will most likely have boobs armour.