I recall that these Humanist organizations and Ethical Societies (both basically atheist) are treated more or less as “churches” being tax-exempt and having officials who can perform marriages and funerals. Basically they’re already churches with God subtracted. Sounds like a religion to me.
That's pretty much what they are and, I believe what they were intended to be. Churches perform social functions in addition to religious ones -- weddings and funerals, as you mentioned, plus social events like picnics, youth and singles groups, day care and preschool, and volunteer and charitable works. A community to provide support in times of grief or economic hardship. And, as this article points out, now expanding into moreal and ethical instruction for children.
For a large chunk of our history, churches provided most of the primary and secondary education, if folks got any outside the home -- and to this day, many private universities have at least a nominal church affiliation, though the church often doesn't have a very hands-on role -- for example, I went to Emory, which is officially a Methodist school. The local bishop has an ex officio seat on the board of trustees, the campus chaplain is always a Methodist (though the assistant chaplain usually isn't), and the school of theology has two academic tracks, a comparative religion track and a Methodist seminary track. Dancing has been allowed on campus since 197, and drinking since some time in the late '60s or early '70s (before that, the issue wasn't so much church influence as the fact that DeKalb was a dry county).
There re a lot of people out there whose faith is lukewarm at best, but who maintain church membership and even attend services because they prize being a part of the community. This is especially true of Conservative and Reform Jews -- Jewish friends I've talked to have to get tickets to attend services on holidays, because space is limited. f the local synagogue had enough seats to accommodate the demand on holidays, they'd have a huge number of empty seats for Shabbat services. Sounds like a religion to me.
I'd say not. Without wading into the debate over atheism is a religion, groups like the Humanists and Freethinkers and Ethical Societies (and, arguable, the Unitarian-Universalist church) draw an audience with a variety of beliefs -- athiests, agnostics, Buddhists and Wiccans, and quasi-believers (for lack of a better term, that's what I've coined for the folks -- and there are a lot of them -- who say "I believe in God, but I don't believe in organized religion.") Even faithful Christians and Jews and Muslims -- thought I'd suspect not many, because they get those social outlets through their church or temple or mosque.
The groups mentioned above accommodate all those folks, and don't take any position on metaphysical matters like the existence of God or an afterlife.