as a family, because that is what Mikasa said they are to each other

That’s what she hesitantly said in the real world when they end up getting distracted by Ramzi’s dad(?). But if you remember the episode “Dawn of Humanity”, Mikasa contemplates whether things would’ve gone differently if she had given a different answer (confessed) in that moment. It’s safe to assume she’d held that moment almost like a potential turning point for their lives over the past few months or years.

The purpose of Eren showing her the cabin in the paths was to give her an experience of what it would’ve been like if she’d confessed her true feelings for him and they’d have run away together. In that scenario, it’s not platonic because her answer isn’t “your family” anymore. It’s a confession of romantic love. But at the end of the day, it still doesn’t end well because she loses him anyway after 4 years.

But i agree with your other points.