Amidamaru got kidnapped by some villain in Flowers. Amidamaru was always gonna have to take the backseat somehow since Hana using the same spirit as Yoh would be boring.
Alumi having a bunch of random spirits isn't plot convenience, she's an itako type shaman which means she can call spirits too her even ones that have passed on. Anna did this in the original a few times and even Yoh would keep some random ghosts around to cheat at schoolwork in the earliest chapters.
If you wanna criticize the series go for actual issues like Takei's glacial pacing.