Let's say I have a +3 Frost Longbow.
Power (Cold) · (Free Action)
All damage dealt by this weapon is cold damage. Another free action returns the damage to normal.
Power (Cold) · Daily (Free Action)
Use this power when you hit with the weapon. The target takes 2d8 cold damage and is slowed until the end of your next turn
A normal arrow fired from the weapon gets the +3 enhancement bonus to attack and damage, and using the power of the weapon, can be converted to cold damage.
Now I want to fire a +2 Dual Arrow from the bow.
Property: When you attack an enemy using this ammunition, you can roll twice and use either result.
Since enhancement bonuses don't stack, I obviously don't get +5 to attack and damage with this attack. But my question is do I still get the +3? Or does the +2 "override" the +3? And if the answer to the second question is yes, that brings up the followup questions: can I still convert the damage from the Dual Arrow attack to cold damage? Can I still use the activated power?
To boil it down:
Which enhancement bonus do you use when you fire magical ammunition from a magical weapon?
Can you use weapon powers/properties with magical ammunition?