> How does the ethical use of this prolematic tool get ascertained? Where and how is the democratic oversight implemented? How is misuse treated and prevented?

I can't speak specifically to this case, but in general when asking a judge for the warrant they also provide compelling evidence that harm would come from disclosure. The judges weigh the rights of the targeted and other parties that would be subject to a gag order against the greater good.

To answer your last two questions, all gag orders eventually expire. It isn't a prohibition against the impacted party speaking out, just a delay. They can go directly to the judge or appeal to a higher court.