Enna is considered really good as a singer, some say best in the branch. Millie and Reimu get lot of praise, and I remember people liking Elira's singing. I think they would be enough.On a more technical note with the cancelled concert, is there any actual professional Singers in EN? I ask because you're trying to sell a Concert. You need Headliners. With Cover, they have headliners in each branch they can structure the "having a concert" around.
As for Millie, she's having an awful time. She's still dealing with loss of her pet and her biggest distraction just got cancelled.