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...
"Thats why I don't have any friends."
...Well yeah, I guess Oshino thinks of the other 4 more as business associates rather than friends. He is all about balance and meeting in the middle after all.
Koyomi isn't a fighter and Monogatari isn't a series about combat. Just beating the other individual down is never the actual point of any arc in the series. Koyomi actually hates the more inhumane powers he has like transforming limbs and such.