When you play an ability or instant ability, is it immediately placed in the discard pile? e.g. If I put Eviscerate on the chain after my own Sinister Strike, can I remove that Sinister Strike from the discard pile as part of the additional cost for Eviscerate?

I think the answer is yes, but wanted to get confirmation.

Also, for effects containing a value of "X" that is not based on cost to play the card (like Sinister Strike), does the value of "X" become fixed on playing the effect or on resolution of the effect?

Thanks,
Corph

Ability cards go on the chain. When they resolve they go to the graveyard (or play if ongoing). Therefore, the Sinister Strike can't be removed, it's still waiting on the chain.

When Sinister Strike resolves choose 1 of your melee weapons. SS then deals X+1 damage. So watch out for instant weapon destruction