"mother" in this story is just a caretaking roll, no gender to it, and everyone in this goes by they/them except Mori and those specified. I'm not sure if anything would be considered a headcanon, but it's up to everyone to form their own opinions and view of the characters in this story. if you think both ryder and miria are fem, that's fine, if you think ryder is masc, that's also fine. at least everyone can agree that they are cute, right? (=・ω・=)
Well I viewed it as more of everyone cares for him and everyone is given the role of mom. But if Ryder becomes a "Dad" later nothing wrong with it, just like if Miriya became a Dad and Ryder a mom. He can be masculine and still be a mom or masculine and a dad there's nothing wrong, I just love this wounderful piece of Universal Art.
Well some of you seems to forgot that men can be mom and women can be dad. As far as I know theres no fixed gender for most aliens so we can just think of them as whatever we like for now. No need to fight each other since implied doesnt mean canon
Also no matter what gender they are, all of them are mom to Mori. They all wanted to be mom because from their understanding mom is the one who loves and care about their children the most
No need to complicate it. All of them are mom, their gender are whatever you want them to be (for now)

It's a correct translation, right? Their married