Hi Alex and Katie, it sounds like you've been bitten by the Savannah bug big time! I think with what you have to work with you are doing the best you can.
I might make two suggestions - first, put a lock (e.g., hook and eye) on the outside of your bedroom door so that you can keep Simba enclosed in there, and second, don't get in his face, especially when he is upset. I know you don't think you are, but trying to kiss his nose when he's just been in an altercation with another cat can be threatening to him.
Whenever there is hissing or growling move slowly (unless you have to move fast to intervene before someone gets hurt of course) and purposefully, while speaking in a quiet calm voice and avoiding eye contact, remove him from the situation. If he shrinks away from being picked up step between him and the others and try to usher him out of the area.
Simba may not appear to be happy locked away in your room, but in reality he probably needs the down time away from the stress of learning to integrate with the rest of the clan. Remember, he has been an 'only child' for all of his life, he needs time to sort out how he feels about his new adopted family. I have no doubt it will all sort itself out in time, so just give him the time he needs to work things out, and of course the same goes with the rest of your cats.