The special girls are in love with you but are afraid to marry you
This applies to only the En. versions of Cute. Look at their reactions, specifically Keria (she gives her uncomfortable look with a bunch of eclipses before "She looks troubled"). They are in love with you but they feel frightened to marry you being that you're both female. You do live in a small country town, though everyone seems open minded. Not that they know though. Plus there's the whole both being female
" thing. In the JPN version this is why they go under "best friends" instead of "wife"..Also because the latter is probably illegal.
The Witch Princess wasn't breaking the fourth wall in DS
She thinks this is all a 'game' to her. She's a mischievous, playful witch so it wouldn't be too far fetched.
These games are AU from AWL or non canon
There's a lot of faults in the timeline between the FOMT, AWL, and DS.
Nina was already dead at the start of the game.
Think about it; it would make a lot of sense if the main character and Galen were the only people to see her ghost. You can already see Harvest Sprites, the Harvest Goddess and several other things to whom/which the rest of the valley seems oblivious, so it would make sense if you could see ghosts too. As for Galen, she was his wife, so chances are he'd be the only person in the valley to see her ghost...alternatively, he can't see her ghost and is simply in denial of her death up until after your child is born, at which point he is able to accept that she is gone.
After all, nobody else (save for you and Galen) ever mentions her or refers to her. Even though she's shown to be the midwife at your wife's labour in DS, Doctor Hardy's there too, so chances are your wife's only ever noticing him and not Nina during childbirth.
The reason why you can't see her anymore after your child is born? People have accepted her death, so her spirit/ghost/soul/whatever has been able to move on. Galen is the only person still referring to her death because everyone else in the valley already accepted it long ago - before you even arrived in the valley - and so it just doesn't seem necessary as part of everyday conversation.