I think they are meant to be twins, yes. They both appear to be the same height and they share the same initial, which is something parents of twins sometimes do.

But then he wouldn't be a femboy anymore

The big hearts aren't implemented yet and likely won't be until Mom's and Mia's stories are brought to the same level as Chloe's and Casey's are. And I believe Mom's story isn't updated past the point where it prompts you to talk to her in the kitchen, but the message saying it'll be coming in a future update was accidentally omitted.

So basically, the answer to both questions is have patience because they'll be coming in future updates.

I think that that next portion of Mom's quest line is coming in a future update, but the message saying so is missing.

That portion looks to be coming in a future update. I am assuming they will come after the dev gets Mia's and Mom's stories to the same stage as Chloe and Casey.

Yes, the save file from 0.1.5 is compatible with 0.2.0. All you should have to do is start the new version and the old save file should be in the load game screen.

I don't believe any of them are active yet. Those are probably coming in a later update.

I don't think the final heart has been implemented for either Chloe or Casey yet. I have a feeling Moraion won't implement them until Mia and Mom's stories have been updated to the same level.