Since possession of Excalibur is most associated with British sovereignty, your champagne villain (who meets no fate) seeking the sword could be Edward VIII
who abdicated in 1937 due to a sex scandal and retreated to the Bahamas after being accused of having pro-Nazi sympathies.
I'd be tempted to set the finale at the assassination, but you're wise to hint at it delicately, as Kennedy recognizes the sword's powers but gives it up anyway.
Excalibur's scabbard was said to have powers of its own. Injuries from losses of blood, for example, would not kill the bearer. In some tellings, wounds received by one wearing the scabbard did not bleed at all.
Then Indy and Marion outdoors on a quiet night some years later (as in Space Cowboys) commenting on Kennedy's space goal nearly realized, cut to the astronauts on or above the moon ditching the sword, and roll credits! Needs a few more moving parts or course...