Nyxathid does work because you don't have to choose an opponent AS (OR WHEN) Nyxathid ENTERS THE BATTLEFIELD, CHOOSE AN OPPONENT so you won't get a counter for every card he has in hand.
No, it does not work with
torpor orb. It only block abilities from triggering -
nyxathid does not have any triggered abilities.
"603.1. Triggered abilities have a trigger condition and an effect. They are written as “[Trigger
condition], [effect],” and begin with the word “when,” “whenever,” or “at.” They can also be
expressed as “[When/Whenever/At] [trigger event], [effect].”"
"603.6e Some permanents have text that reads “[This permanent] enters the battlefield with . . . ,”
“As [this permanent] enters the battlefield . . . ,” “[This permanent] enters the battlefield as . . .
,” or “[This permanent] enters the battlefield tapped.” Such text is a static ability—not a
triggered ability—whose effect occurs as part of the event that puts the permanent onto the
battlefield."