Keep in mind that the cops don't have to provide you with their reasonable suspicion in order to demand ID. It's not until court that they have to provide their reasonable suspicion. So they have plenty of time to come up with justification after the fact.

Also, on the Fifth Amendment I thought I had read somewhere about a case where a man simply remained silent and never once invoked his right and it didn't end well for him. I cannot remember the details, but for some reason I thought that you still had to invoke the fifth even if you have not yet answered any questions. I'll have to look back into this later and post back if I find the story.