Pretty common scenario, but here goes: Webster accepts the Vice Presidency in 1848, becoming President on the death of Zachary Taylor. If Webster dies on schedule in 1852 (considering the stress of the presidency, possibly earlier), President pro tempore William R. King succeeds him. If King's weak health coupled with stress causes him to die, would Speaker Lynn Boyd become President, or King's successor as Pres pro tem?
Any ideas for how the effects of this strange set of circumstances will be? Presumably the Compromise of 1850 will still be passed, but could a proto-25th amendment be passed?