There are ways to come to grips with such continuity errors, despite the fiction of the whole of it...From the Greek mythology, to the biblical texts about God,to resurrection, death and rebirth, to modern marvel, dune, charmed, and others there is a mechanism for someone, a hero, a godlike being/ a God , a Navigator, a wizard , etc to fold the universe and travel in time and in space...Xena and Gabrielle went to Hell (met Mnemosyne there) and back again as the mythical Hercules traveled to Hades and captures Cerberus and Ulysseus travelled to see his mother and Achilles (who was in Troy and then in Hades) and Christ like Lazarus rose from the grave (or Kratos in God of War or Dante and Virgil in the Divine Comedy)....Considering that Hades, Hell, Styx, Lethe, etc can be easily assigned to different locations on the planet and the mechanism of death and rebirth, Xena and Gabrielle travelling to different times/locations means that , like the mythical heros, they died (travelled to hell) and were reborn...
Death in the ancient World was not a feared process (people have sought immortallity since ancient times but death was and is a certain unavoidable thing), heros were willing to go to battle to earn the place in Elysium or elsewhere...And it was an "explainable process".... Today we are taught to fear death, back then they were taught to face it heads on.... Different mindset, but people of war like Xena and Hercules would have that kind of mindset.... Thus Xena and Gabrielle in the time of Ceasar or Christ or other eras are divine heros having returned from Death...A sense of immortallity that can actually be eternal hell, as eternity is impossible to grasp for most beings....