How does banding work when a creature with banding attacks in a band with one creature that doesn't have banding and a creature with banding blocks that band along with at least one other creature (also without banding)? Does only the creature that was chosen to be blocked have its damage assigned by the defending player, or does the entire band have its damage assigned by the defending player? Likewise, does the attacking player get to assign the damage of the banding creature even if that creature chose to block the creature without banding?

Both attacking creatures have been blocked by both defending creatures, so all creatures are blocking or blocked by a creature with banding, so, for each creature, the opponent of its controller gets to assign the damage dealt by the creature, and can ignore the creature's damage assignment order when doing so.

Band 1 consisting of Banding Creature + Non-Banding creature is attacking. It is blocked by Banding Creature and Non-banding creature
Both attacking creatures are blocked.
The defending player will divide the damage the attacking creatures assign as he or she chooses among the blockers. The attacking player will divide the damage the blocking creatures assign as he or she chooses among the attackers.