boltnrun

You're in high school, so I get why you think it's possible to have an "online boyfriend".  It's actually not possible.
You're also very young and your home life seems unsettled to say the least, so I can see why inventing drama and stories seems exciting.  
You are not going to ever meet this boy (if it is in fact a boy!) and you are not going to move to be with him.  And it's highly unlikely whoever it really is will turn out to be a murderer!  It's more likely that it's an adult predator or some kind of scammer.
Please tell a trusted adult (Your aunt?  Someone at school?) about your online activity.  Also tell a trusted adult about your father's disturbing activity.  

My brother wasn't on drugs when he chose to go along with his wife in removing our mother and I from their lives. He was just completely enthralled with her. He's the type that falls hard and fast and becomes completely enmeshed with whoever he's involved in a romantic relationship with. I learned I needed to walk very carefully with his second wife because I knew he's capable of cutting me off if his wife tells him to. He eventually learned on his own how negative an influence these women were on him. Which is why he's twice divorced, sadly.
It's actually very common for parents to keep their adult children on their company medical plan until they age out (usually at 24, I believe). It usually doesn't cost much more to pay for a family plan than it does for an individual or a couple. If it were anything other than medical coverage (such as rent, car payment or money for leisure activities) I would say yeah, cut it off. But medical coverage is a different thing IMO.
I hope things are resolved for you soon. 

While it's lousy that he is apparently taking up with your so called friend, it may be a case of your efforts being too little too late.
I presume he's told you more than once that your talks of breaking up, your inability to show him love and your anger and snapping at him were upsetting him. But if you did nothing about it in eight years it's not surprising he's not particularly eager to give you even more time.
I think removing yourself from the situation and attending counseling to work on your issues are great ideas. Even if it doesn't result in him wanting to reconcile, learning how to relate better to others is a good thing.