Complex sentence: Lionel discovers that Lex died in this world sometime after his own counterpart, and posing as his own doppelganger, attempts to make amends for favoring Clark's counterpart over his son in his reality.
Split into simpler sentences: Lionel discovers that Lex died in this world sometime after his own counterpart. Posing as his own doppelganger, Lionel tries to make amends for favoring Clark's counterpart over his son in his reality.