I think part of your problem, outside of probably having read a few spoilers before hand, was going in thinking "this is a horror movie" instead of "damn this is a real life situation, and what would it be like to be a black man in this very situation" which for someone like me (a black man), is a very real situation to be in.
The beauty of this movie, is that placing yourself in the main characters shoes, as if this is your life, you would probably have been stuck in the exact same situation he was, when he was, and how he was.
as far as how the consciousness worked. For one, he was hypnotized before he even realized he was being hypnotized. Once he had fallen to the sunken place, he was already ripe for transfer as all that was required was to be able to take the dominant consciousness and place it in to the back of the mind, or the subconscious. basically making you a passenger in your own head while the new transplanted brain/consciousness is placed in the dominant position. The reason for keeping the old consciousness is that the brain is very complex and having a subconscious that is already attached to the body allows the new consciousness to just drive, and not have to relearn how to walk or talk.
as far as the family goes, no one goes into a situation and thinks, this is a horror movie, I should just murder my new girlfriends family.... that would be crazy. don't go into this movie thinking it's a horror movie. It is a "keepin it real" thriller. The family interactions were awkward as they were kinda try hard in some instances to make him "feel comfortable". In a real life situation, you would probably recognize the awkwardness and try to brush it off, and keep taking note of repeated awkwardness before you finally had to let your girl know that this **** was just too awkward for you, which is basically what happened.
But at the same time, I don't expect everyone to relate. It really was a Black Horror, which was most emphasized by the scariest moment in the film when the cop car shows up at the end with lights flashing, and the ENTIRE audience in the theater gasped thinking that after all this bullshit he just went though, he was either about to get shot and killed on site, or arrested and found guilty for everything that went down at that house while they all think he's just some crazy *****. luckily that wasn't what happened, but it was honestly the scariest moment of the whole dame movie