Kouko is certainly strange, but it all makes sense based on the earlier eps. She was strange before the rejection in the extreme stalker way, now she's without a husband target, so the craziness is bursting out in different ways. I found it quite cute, like you said, that while she's emphasizing "friends" all the time, in many ways she's still acting like the girlfriend she has never been outside of her delusions. I'm somewhat happy she didn't simply switch from Mitsuo to Banri. It would have felt a bit hollow and this way she's actually showing character growth. On the other hand it's a bit cruel toward Banri, but I wonder if she's got a clue at all about that detail. Surely she does realised, at the very least, that she hasn't given him any sort of answer, but the longer she keeps repeating "friends", the more that will become the final answer, no matter how understanding Banri might be.

Still, Banri has other things to ponder about right now, so that will buy Kouko time. I just hope there won't ever appear too heavy a triangle. Perhaps the OP indeed told without doubt how this will end making it a useless worry, but I would really feel sorry for Kouko if Banri now switched to Linda for the sake of his lost memories and the ghost hanging behind his shoulder.