I don't know about the others but the dream about you going to Adam's house and you can hear the water running and he sounds like he's underwater, is possibly a sign that perhaps you have some unresolved feelings about him. Maybe you feel like there was something you could have done to stop him and that perhaps he would still be alive.

Also, perhaps the dark cave is a image of your search for meaning and the truth. The truth about your brother and other things. The creature or whatever it is that is in the cave is your concious mind trying to keep you from finding out the truth because it may be too painful. But I really think that you need to find out what your subconcious is telling you or you will continue to have these nightmares.