They know, but how much hey know is arguable. I don't see how they mentioning it automatically means they're experts on it. Clearly if they were it would have already been in their hands long ago, whatever it is.
Yes, but look at this conversation you provided. "What is his connection to Reyleigh?" - they have no clue. Then "I seriously doubt he'd return to the stage after so many years." - a guess as good as any. Just an assumption. Who's to say he won't return or get mad or whatever? A lot of these talks are just them assuming things and exchanging opinions to some big events happening. It's not like it's inside inside inside info
It's weird that they feel some are dangerous and some not based on doubt - but that is exactly what I meant to say, they don't know everything and they are mostly driven by fear due to lack of information or predictability. They KNOW Reyleigh and are brave enough to predict his moves (I think this is a mistake btw, if it was me I would arrest them all hehehe), they don't know the Straw Hats as well and there is a threat. Because they don't know what Robin would do. But we know she wouldn't use a "weapon" to hurt anyone, so I wouldn't say she's so dangerous as they pose her out of fear.