I've debated this particular topic several times before, though I'm not sure that it really merits any debate. However, since you go about it this sincerely and meticulously,
evangelicalhumanist, I'd like to point out that your own examples actually proves you wrong:
evangelicalhumanist wrote:A belief that God requires attendance at mass, or you’ll go to Hell, is likely to lead to one going to mass. On the other hand, lacking that belief does not mean either that you will or will not attend mass.
Surely it will make it less likely that you will attend mass? People who believe in God often pray to God. If you don't believe in God, you have no reason to pray. This difference in behaviour is clearly related to your belief, whether it's a theist belief or an atheist one.
evangelicalhumanist wrote:The belief that mushrooms may all be poison is likely to prevent one from eating mushrooms. But not believing it will have no effect whatever on whether one eats mushrooms.
Surely it will make it more likely that you will eat mushrooms? Whether unfounded or not, your view on this is bound to affect your actions whenever muschrooms are being served.
evangelicalhumanist wrote:The atheist – in my view – is not “informed” by a lack of belief in god.
Had you never heard of God or been aware of the concept of religion, I would agree. But since other people practice religion in all manner of ways, their way of life is obviously an option. A choice not to do something is also a choice. Does your lack of belief in God not inform you not to pray, go to church or follow religious dogma and such?
evangelicalhumanist wrote:In my firm view, because the idea of “god” plays no part whatever in how the atheist lives his life, then atheism is itself not a form of belief.
But if that was true (that the idea of "god" plays no part in your life) surely you would have no need for the word "atheist" in the first place? The fact that you are confronted with other people who believe in God makes the idea play a part in your life. Again, the choice
not to do something is a choice too. It seems to me that your only point is that if there were no theists, there would be no atheists either.
evangelicalhumanist wrote:That "lack of belief" is as barren as barren can be, and therefore the very antithesis – as I said at the beginning – of what belief is about.
Since you started this topic in the first place, I would guess that you feel quite strongly about this "lack of belief" which makes it anything but barren to me.
Having said all that, I'm still not certain that this topic is really worth debating. Theism and atheism, in all their guises, are positions, points of view, attitudes and, yes, beliefs. It seems like you have something against the very concept of "belief" that makes you want to distance yourself from it.