Foot bone further supports that A. aferensis walked upright

Analysis of the fourth metatarsal remains of A. Aferensis individuals now seem to provide solid evidence that A. afarensis habitually walked upright. The fourth metatarsal bone was found to have a 'twist' such that when one end is attached to the toe the other end rises up, this torsion is found in feet with an arch. This also shows that 'Lucy's' feet had evolved away from the flexible midfoot found in apes, that enables them to grasp branches.Exciting news! Maybe not as exciting as the discover of a new species of humans but close.

You're right that there is still a good amount of debate on why/how humans evolved bipedal motion. Some of the hypotheses that I recall were the thermoregulation model, the behavior model, food-transportation model, and the savannah hypothesis. I also think there was the climate model. oh yes, the aquatic theory also.As far as whether our common ancestor with the chimps was functionally bipedal or arboreal, it would seem evident that it would have been arboreal yet, like chimpanzees today, spent time on the ground foraging. After the split, bipedalism developed in our lineage. I would say no, for the simple fact that fossil evidence suggests that we evolved from an arboreal ancestor, and I don't know of any evidence that suggests apes reverted back to knuckle walking after being bipedal. Iirc A. anemensis's foot bones were more primitive than A. aferensis and still retained features that allowed it to be arboreal.