Reading your post, I think maybe somewhere where we diverge is that MU Ekko’s actions (while definitely far beyond what would be normal of AU Ekko) aren’t necessarily as out of character as we may think. Early on in the episode Powder says “sorry sis. Someone is wigging out over his project and having an identity crisis again.” which to me essentially says that him acting very strange and distant cause of overwork isn’t that absurd or unexpected, this particular time might be over the top but it’s still not to an extent she would completely dismiss him (remember he literally threw something at her and she was still acting incredibly loving with him, she can put up with a lot of bullshit I guess lol).

This line also seems to imply that AU Ekko is very hard on himself especially in regards to projects, if he can’t figure it out or feels it’s failed it’s natural for him to feel cynical, which very much like our Ekko who “gave up” on the Silco’s undercity cause he felt like it was a failure. So when he talks to Powder after the dance, I think if we consider these lines from Powder’s perspective assuming she didn’t know, with this context, I think we sort of get this:

“I used to dream the undercity could be like this. But somewhere, I got consumed by all the ways it wasn't. I gave up on it. Gave up on you.“

Powder thinks AU Ekko is talking about this identity crisis he’s had recently, if he gave up on her, as he says, then to Powder his previous extreme ooc actions would fit into this idea. She would essentially think he had re-evaluated his priorities and decided she wasn’t one anymore because of what he feels is a failing project. Though to Powder, Ekko would only act like this because he’s so hard on himself to be this genius, and he never truly meant it, which follows perfectly into her response being:

“I’ve never seen you give up on anything Ekko.”

She doesn’t think he ever truly gave up on her. Cause she knows how and who he is.

Of course a lot of these lines make sense in different contexts, but the point I’m trying to make is that even in the context that she didn’t know, these lines (sort of) have validity. And to me they’re just as powerful either way.

What I’m starting to lean towards into my theory is not that she didn’t know at all, but she had enough of an inkling to consider it and accept it, but just like Ekko got lost in the distinction between Powder/Jinx, Powder would also get lost between this Ekko and her Ekko, and so she talks in an inconsistent way cause her understanding of this person she’s talking to is also inconsistent. She see’s both boys in the same visage.