I dropped my bag means that you were holding your bag and let go of it. My bag fell down means that you were not holding it when it fell, for instance, if you had placed it on a table but it slipped off. Hi all (or "hey y'all" in the southern USA)= very informal, maybe to greet friends that you run across Hello everyone = a bit more formal, a good way to start a presentation at work for instance. Hi everyone = "hi" is pretty much always casual

To add to this, in England we would pretty much never say 'hi all', that would seem quite unnatural. I concur thought that Hello everyone would be used in a presentation environment, where you don't know all the individuals. Again though in England we might use hi everyone amongst friends/acquaintances to greet the group but it's not a sufficient enough of a greeting so we would then go on to greet people we have a closer relationship with using phrases like "how are you?", "how's things?", "what's happening?" "Are you ok?" etc. confusing I know, but in an inforal setting saying hi everyone is good as rule and then further greet people you have a closer relationship with.

The prescriptive difference is that "bag" is the receiver of an action in the first sentence and in the second sentence, it is the subject. So by saying "I dropped the bag" You are the doer of the verb and the bag is receiving the action from you. In the other sentence, the bag is the subject and is the doer and there is not object receiving the action. "Hi all," would be understood but not used with frequency. "Hi, everyone" seems more natural.