I'm starting to wonder if we should break the "weapon protrusions" out of the Malleable Form SQ.
The whirlwind reminds me a bit of a barbarian's rage. So let's figure out the benefits: +2 AC (maybe from a +4 Dex bonus), 4 attacks with boosted damage. How about this?
Whirlwind Rage (Ex): When pressed, a busen can enter a berzerk state for 1d6 rounds. It gains a +4 Dex bonus and sprouts weapon protrusions all over its body. It can make 4 attacks at its full base attack bonus and move up to its full speed as a full round action, and each attack deals 2d8+4 (2d8+Str) damage. After its whirlwind rage ends, the busen is exhausted (fatigued?).