After three years of delays, confusion and overspending, the Korean War epic "Inchon!" may finally make its way onto domestic screens. MGM/UA has picked up American distribution rights to the troubled film of one of Gen. Douglas MacArthur's crucial battles, which to date has shown in this country only once, at a Washington benefit for retired Navy personnel. Innumerable problems kept the film from a distribution deal: Its budget soared to a reported $30 to $40 million, scenes featuring the late David Janssen had to be edited out and others with star Laurence Olivier reshot and -- most crucially -- controversy arose over the extent of the participation by the Rev. Sun Myung Moon's Unification Church. The revelation that the church had financed the film stalled its release for some time, until MGM/UA made its deal following a screening at the last Cannes festival . . . Producer Frank Yablans has said that he sidestepped Vatican authorities when it came time to film a crucial scene in his upcoming "Monsignor," a tale of corruption in the Catholic church. The scene had to be shot in St. Peter's Square; so rather than endure any red tape, Yablans simply moved the cameras in on a quiet Saturday morning, shot the scene and left town.