A:AnswerPretty much any GoPro can be used as dash cam, but your best options will probably be one that offers a looping feature so you can record indefinitely, and just save any segments where activity you want to keep occurs. Take a look at the HERO5 Session, the HERO5 Black, and the HERO6 Black. Each of those has a looping function and may be just what you're looking for.
A:AnswerDefinitely. I've been using them for years and have never had an issue with them coming off. Just make sure you match the mount with the curve of the helmet, depending on where you're putting it. Get the entire surface of the mount tight against your helmet.