So I reveal a guarded objective and I follow it with a treachery card. If I use Eleanor to cancel that treachery card, the replacement card - is not guarding the objective right? The objective is left unguarded, with a discarded / canceled card, right?

The objective is now unguarded. First card was revealed and became the card guarding the objective. You cancelled its When Revealed effects but it still fulfills the objective's Guarded requirement. The replacement card won't guard the objective.

That's a pretty good trick! I guess you could do something similar with guarding enemies and Quick Ears.