As you said, both text are different "After your hero attacks an enemy, …" and "After you use one of Nova’s basic powers, …", however both text refer to the same timing point, both happens at the same time, the same happen if you defend, both trigger at same time "after the hero defends" and "after a hero uses a basic power".
Sadly, this is not spell out quite bluntly in the rule book reference 1.6. The closes thing is this section:
In the rest of the book there are no mention of a response of using a "Basic Power" in any timing sequence (see last image), only references to the core use of said basic power: Recover, Attack, Thwart, and Defend. I understand that changing the current engine is something more difficult to do than fixing a card bug. Maybe this is not the highest source of confident, but asking around Discord servers, people of the community sims to agree with my assessment.
To complement on the sequencing, in the same timing point we are discussing here, abilities labeled "After a [character] is damage" and "After a [character] is defeated" happens at the same.
Regarding your Shield Toss example, I get that Janbjorn trigger after the resolution of the event since it is a single instance of an Attack, regardless of how may cards are discarded. Will Janbjorn fire 3 times after playing Dance of Deth since it is consider as 3 attack instances??