Okay, question two: Can a Unity (Free or Pro) project be licensed as Open Source software?
The main considerations would be ones that apply to your code regardless of Unity, including if you own it, and the terms of any code you link to (for instance, if you code depends on GPL'd, you must distribute it as GPL too).
As long as you purchased all the necessary tools legally, I don't think that this has anything to do with Unity.
There is also the issue of redistributing the source code of the standard assets... the copyright might get messy if you changed the code.
Do you know what licence the standard assets are?