1/ The staff of the venue is not the police. That makes quite a difference.
It is THEIR property, not yours. When you get to the door if they have a policy that you don't like, or you don't want to leave your prohibited gear, check your coat and hat or what ever you don't like then turn your happy butt around and leave. In their house you play by their ground rules. By choosing to enter you have agreed to their rules. They are not going to leave their butts legally hanging out because of YOU. And if you choose not to follow their rules once inside they can kick you out.
2/ I have been arrested out of my vehicle and the police did not search my car. They did not have a search warrant and I didn't give them permission. I may not be a lawyer but I certainly talk to mine.
Your lawyer is not the Supreme Court of the United States. The Supreme Court long ago ruled that Inventory Searches are lawful when a something like a vehicle is being seized. Here are the exceptions to the 4th Amendment.
Search and Seizure Guide
By the way, I'm no lawyer either, but with 27 years currently as a LEO I have dealt with a whole lot of them. They call it
Practicing Law for a reason. If they had it right we wouldn't need as many as we have.
3/ What prohibited items? Did it say on the ticket that SLRs were not allowed?
Does someone have to tell you when to take a breath in and when to exhale? Do they need to put on the ticket that you can't bring in your illegal drugs, guns, bombs, pets, blowup dolls and lube, stereo systems, lazy boy recliners, fireworks, power tools, stove and food to make your own... etc...etc...etc??? You bought a ticket to see a show, what did you really expect you were there for???