Damage dealt by creatures and the order of damage
All combat damage is dealt at the same time, so yes, it would have been a draw.

Deflecting Palm is a replacement effect (instead of your opponent, you take damage the moment combat damage is dealt). This happens at the same time for all combat damage. So, the Polukranos damage hits you the same time the Arbor Colossus damage would have hit your opponent.

/edit: last paragraph was incorrect