Why?

A child has limited power compared to an adult. An adult has limited power compared to a god.

Similarly, a bodhisattva —whether fallen or not— has more power than an intellectual animal. Their actions have more power, and more consequences.

Are fallen bodhisattva subject ordinary karma as well as Katancia karma, in proportion to their prior level of development?

If they are subject to Katancia karma, does this mean that the negative karma they acquired is negotiable if they once again commit themselves to the path?

Yes. Fallen bodhisattvas have regular karma as well as karma for the gods.

Karma is negotiable, even for the fallen gods. Those who return to the path can be shown great mercy if their repentance is proved, tested, and confirmed through tremendous sincerity and actions.