Does cancelling a treachery negate Forced effect like Thardir's? Then Eleanor breaks BoCD a bit.

I believe the forced effect still happens. I seem to remember on one of the CotR podcasts, one of the designers brought this up as a way they were making encounters with effects that bypass cancellation effects.

Does cancelling a treachery negate Forced effect like Thardir's? Then Eleanor breaks BoCD a bit.

how does his force ability read?

Eleanor and Test of Will only cancel the When Revealed effects of a card (Treacheries for Eleanor and any When Revealed for Test of Will), so if the card you're trying to cancel has a Forced, then nothing is cancelled. In Thaurdir's case, the cancelled Sorcery resolves with no effect and can trigger Thaurdir's Forced.

I think if you use The Door Is Closed, Thaurdir won't be triggered since you cancelled the whole card. The game behaves as if nothing was revealed, although it still satisfies the requirement of revealing a card during staging. Right? There is an entry in the most recent FAQ about that, not at liberty to look it up at the moment.

Eleanor or A Test of Will only cancel the When Revealed effects. A Sorcery traited card was still revealed, regardless, so Thaurdir will trigger if you use those cards to cancel.