Does fasting the first nine days of Dhul-Hijjah suffice for Hajj, and does breaking the fast on one of those days invalidate the fasting of the remaining days?

Fasting the nine days of Dhul-Hijjah is legislated, but there is no evidence that fasting them is equivalent to performing Hajj. Fasting each of these days is an independent act of worship. Whoever fasts a day will receive its reward, and whoever fasts more will receive the reward for what they added.