[masaegu]

I am pretty sure I hear 「危なさそう」 more often than 「危なそう」 around me in Tokyo, but I am also certain that grammarians would only consider the latter to be correct. I actually use the former myself in conversation if not in writing.

「危ないそうじゃないです。」 is grammatically correct but it sounds very unnatural. We would prefer saying 「危なくはないそうです。」. The comparative particle 「は」 can be dropped.