Doggs- Negative atheists have the position that there is no reason to believe in God unless evidence is provided for him.
You're right, saying there is no reason to believe God exists is not a metaphysical statement, it's a statement about existence, but it assumes contingent metaphysics. You've actually helped my point, the fact that the negative and strong atheist statements are different, yet have the same metaphysics is because they are not metaphysical statements.
Ballin- The unicorn's necessity is 50/50 until we've examined it. The reason why it's not an issue is because it's undoubtedly clear it's not necessary.
Nevertheless, to answer your questions about why the unicorn is not necessary, it's the same reason as why Zeus is not necessary. The unicorn has a specific form, is physical, so it presumes the prior existence of at least space and time, and anything which has a cause is not necessary, because there is a prior reason for its existence, etc. . You can contest these points, but that would be petty and missing the point of the debate. My point is not that God is necessary, but rather that the default position should not be that He isn't.