Mikumo's character could have been handled better throughout the 2nd half as a whole, but I'm not sure what more they could have done for her here in this final episode alone.
She looked
glorious. She sounded glorious. She eventually managed to change back to Walkure!Mikumo, under her own power. Yes, she needed to hear Freyja reaching out to her in order to do it, but she still did it. And it all ends with a happy tearful reunion between Mikumo and the three other Walkure members from Episode 1. I'm sure Walkure understands that Freyja wants to be with her new boyfriend right now.
Yeah, the mystery side of Mikumo leaves a lot of unanswered questions, but hey, Mikumo does say "music is mystery".
Mikumo being mysterious is simply
her, just like Freyja being genki is simply
her.
But the whole Lady M thing was a silly tease that went nowhere, so I'll agree with you there.