Miko's heal seems fine to me in endgame, considering a full MAG build (I don't see any reason to build her as a tank regardless). It might scale badly before that, though...
Alice's damage should be good so long as you're using Sorc and Marisa. Hanged Hourai Dolls is over 300% with row attack enhancement and she gets +30% off Marisa and +32% if you can activate Final Blow, and those are some winning numbers- she ignores a hunk of it's high mind factor and it's not a big problem in postgame in general. She does need final blow to -really- keep up with competition, unfortunately. Her enormous eva should be good if you're trying to use Mystia, tho.
Mamizou seems... average. HP and damage reduction is king in postgame, and she's got both, but other characters just... do it better? Her only real specialty is attaching any weakness on any skill, main or subclass, which is... convenient, and can combo well with elemental boost passives, but I'm not sure how much that really matters. Her stat layout doesn't work that great until endgame, so she belongs in postgame where she is.
Futo mostly comes into her own at like, lv500+ when she can afford her whole awakening set. I only barely decided to replace her with Mokou, it's easy to maintain 10 plates and she gets a helluva lotta boost from it. She's really averagey before awakening, though.
Still doubtful about Yuyuko having endgame potential. My characters with high level/library cost have depressingly poor stats, other than Miko possessing Maintennance. Eirin is still king because of overheal, but man are her stats garbage; it feels like those sorts of characters will only keep working if they have supremely good passives/skills, which to be fair, many of them do! e.g. Yukari.
A character I'd be curious about in NG+ is Tokiko. Her "regen when a sub-200 affininy is attacked" is good in maingame but useless in Plus. Try to activate reading once you can afford the buff sustainer, then use subclass support with her mp regen on hits. Koishi might work alright too since she can achieve eva scores that are absurd in maingame, even if her gimmicks are too expensive to learn.