Xena and Gabrielle must have sneakily been at the ambrosia at some point...They'd have to be immortal in order to be present at events in history that happened up to thousands of years apart. Despite the fact that Xena gets killed off in the end, so it must eventually wear off...
The episode Bitter Suite takes place entirely in The Black LodgeWell, it explains the strange reality-warping nature of the place, the strange distortion of various characters, and the spontaneous musical numbers. However, this is an older version of the Black Lodge before it was owned by the Little Man from Another Place, which is why he doesn't appear.
Callisto's entire motivation is that Xena will fix Callisto's life again.Xena ruined Callisto's life and Xena can fix it again! Well, at least that is what Callisto feels. So she torments Xena all the show until it works! at one of the final episodes, Xena really fix her and makes her an angel and send her to heaven!
The double standard of The Twilight of the Gods was intentionalAs pointed out in the main page the One True God having Xena kill the Pagan Gods was kind of hypocritical. But the God of the Old Testament was petty, jealous, and angry. Just because Eli and The One True God are supposedly good, doesn't mean they are. Xena never intentionally sets out to kill the Gods they go after her.
Gabrielle's emotionless state was from Bleach fumes.