I'm not so sure about the idea of Steve Carel as James Hosty. As for JFK and Jackie O, I think they would have more impact if they were limited to TV, newsreel and documentary footage, and for the scenes where they have to take part in the movie JFK's role should be more implied than heard or scene. I just think it gives him more of a presence and lend a sense of mystery to things if it's just a scene with Jake talking into a phone and listening to JFK yet we never here the other side of the conversation. As for Jackie O fans out there, well, I hate to say this, but she's one of the elements you could cut out without losing much. Sorry.
As for the most important role of Oswald, I think Doug Hutchinson could do it. I also think Jeff Demunn would be a good Deke. Plus, they are both King veterans in a way