Re: Could have been is a past form of could be?
I think it's more simple than that.
A: Peter, look at this sign. It says the show starts at 7, but doesn't it start at 7:30?
Peter: Well, the sign could be wrong.
He said that the sign could be wrong.
A: Peter, the program says the show starts at 7. I was sure when we checked the Web site before we left that it said it started at 7:30
Peter: It's always possible the Web site was wrong.
Peter said that it could have been wrong.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.