Assassin’s Death Strike Critical?

This is a question regarding the double damage of the Assassin’s Death Strike.

Let’s say a 20th level rogue uses the Sneak attack feature on someone, it is a critical and it fails the CON save for Death Strike. Would the damage be an additional 40d6?

Follow up question. Say that the Rogue in question had purple worm poison on the weapon in which it made the attack. Would the both the critical and Death Strike apply to it, or only one?

Another follow up question: Say the Rogue was using a Giant Slayer Shortsword and the target was a giant. If the giant failed the DC for Death Strike and it was a critical hit, would the 2d6 damage dice be doubled or quadrupled?