This is especially so since she seems to have been on the verge of starting a new novel-only series, Challenger, that was apparently already plotted out. Despite this, it all came to a halt.
That's misunderstanding how this works. The series was never guaranteed to exist. It got a "pilot" as part of New Earth
, and because the timing was right, they included it as part of the following year's Gateways
crossover to give it a further push, but those were just tryouts. In order to go beyond that, the first two books had to sell strongly enough to justify doing more. And evidently they didn't.
A pity. This explains the large gap between Rihannsu novels, too, right?
No; Diane Duane did write other Trek novels during the Arnold era. The reason the Rihannsu books weren't continued is because TNG's depiction of the Romulans overwrote them. They didn't fit canon anymore. When Swordhunt
was originally published (in two volumes), it carried a disclaimer making it clear that it was revisiting an old, alternative take on Trek continuity.