A sphere a hundred light years in diameter would be an almost insignificant speck within the galaxy.
Not for Spock, or for any of his audience. Essentially, it would amount to All Known Space. And even Spock plays fast and loose with such terminology from our viewpoint, using "world" in the sense of "universe". Commodore Barstow in "Alternative Factor" is happy to consider "the entire galaxy and beyond" his jurisdiction, too, despite this being very unlikely to be factually accurate.
I fail to see how a black hole could stop the supernova once it's already expanded to engulf other star systems.
Indeed, which is why it is natural to assume that it only ever engulfed one star system (thus necessarily the Romulan homesystem) before Spock deployed the red matter.
The question this begs is, what did Spock hope to achieve with his red matter originally? If it can suck in a supernova that has expanded to beyond the orbit of Romulus, wouldn't it then automatically also destroy Romulus if deployed as originally planned? Clearly, Spock did not intend
to destroy Romulus to save the rest of the known universe - such destruction was "unthinkable" to him.
Did Spock originally intend to use a lower dosage, perhaps? We see red matter create black holes of varying sizes and strengths, so such fine tuning would probably be possible. Perhaps a smaller drop, administered earlier, would just kill the Romulan homesun and give days or weeks in which to evacuate the planet. Or perhaps a very small droplet would turn an impending supernova into a far more slowly dying type of star, giving an evacuation timespan of decades, centuries or millennia.