Question 9: Is it absolutely required for a Half Celestial/Half Fiend to maintain there alignment axis? (Celestial have to be good and Fiends have to be evil?) I have yet to see a rule other than in the Monster Manuals description for them that they cannot be evil/good. What if I as a player am playing one? Do I take any penalties for playing them against their normal alignment?
You can be the opposite alignment of your alignment subtype, with no real penalties (other than possibly RP ones). However, there are some potential negative effects for alignment-based spells and such.
You get to keep the original alignment subtype ([Good] or [Evil], depending on which one you choose), while actually being the other alignment. This interacts in a rather complex manner with alignment-based effects (depending on if it affects non-Good or non-Evil creatures, or affects Good or Evil critters), as you count as both as long as you maintain your alignment.