"Religions are mutually exclusive systems, since the primary assumption of these systems can not coexist with a differing set. How can a society function, when thousand different religions scream to set policies and regulations?"

In my country, we have the first amendment to the constitution which says: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof." This phraseology guarantees freedom of, and freedom from religion depending on your perspective. As far as I'm concerned this makes the State more agnostic than it does atheist. However, we're talking about the State here and it's a fictional entity, so, if we ascribe human traits of belief or non-belief to it is an anthropomorphism. If we anthropomorphize the State, in essence, we are making the State a God are we not? And it might be a very jealous God if it "feels" the need to ban all religion. I think an argument can be made that agnosticism is neither belief nor disbelief. I think it's probably most accurate that fictional entities neither believe, nor disbelieve in anything. Whereas atheism is the (religion or) belief system that there is no God.

"Religion is already outlawed and censored from public consciousness in humanist secular democracies, other than as a personal fad or philosophical preference."

If we're talking about America, I don't think so. We have a lot of people from many different religions here. Just so long as they don't break the law or harm people while practicing their religion, it's perfectly fine.

"Would formalising such arrangement in legal statutes be too much of a hassle for humanist democracies?"

There are a lot of people who derive great spiritual benefit from their ability to freely worship. To curtail this in America would cause great upset among the masses. Also what your talking about is banning ideas and the legal right to speak about them. That's a very slippery slope to go down. Once you start, where does it end?