Mystia is fine for bosses. Her single target is pretty fast and has respectable damage.
I'd question the lack of Nitori more then the lack of Eiki, as Nitori is much more powerful for a lower SP cost and much higher usability in randoms. Only worse in the sense that Nitori can't damage billion def enemies; Megawatt pierces enough defense to still stand up to Last Judgment in other cases. Nitori is just grossly overpowered in general.
One other problem with sticking with a single team of 12 the whole game is scaling; some people are really bad early on and really good later, and vice versa. This is partly due to SP costs (Especially for, in your party, Utsuho/Kanako/Yuyuko, and to a point Mystia as her multitargets are 80 sp), and partly due to how stats scale in general. Partway through with that party I think you'd start to encounter SP problems trying to deal with the random battles. As well, taking along Minoriko is -immensely- helpful for maingame, while being droppable in Plus.
Eirin is pretty bad until late in where she can be a defensive nuker with Astronomical Tombing (her heal only covers 50% hp and is very slow, when a single-target heal should be capable of fullhealing or near so), Utsuho is questionable overall because she's actually not very strong nor does she have good defenses, and Yuyuko, while being an awesome defensive nuker later, is difficult to make use of before then due to slug speed and low damage, especially due to the amount of bosses with a SPI resistance.
