I would probably drop the Daemon prince. Typically when I run them I leave them alone so that my opponent doesn't pay too much attention to them and I can use them to maul a weak spot. In this case I feel like that strategy won't work and the only thing saving it from an early death is the helldrakes. If you want to assault then drop a psychic level and get the black mace, if you want to keep it flying then grab the burning brand so it can pretend to be a helldrake.
In regard to your questions,
IMHO unless you need the heavy support space for something else then Obliterators are always well served by being by themselves, they can't all be shot at the same time, they can choose their weaponry independently and so long as you position them properly range is never an issue. And they don't really benefit from having a character joined to them either. I really don't know about the cultists, I get it for fluff reasons but I don't see them working as standard cultists. I typically only bring them as plague zombies in groups of thirty. If you do want to run them then 2 groups of at least 15, because if your opponent makes a point of trying to wipe a squad then 10 men isn't going to offer any resistance at all. Otherwise generally speaking there isn't anything useless in this list and overall it's pretty good. I'm not a fan of vanilla marines but that's more due to personal taste really.
Good luck at the tournament though, I'd be interested in hearing how it goes.