This is not true. That's not how things work physically.
This is true to some extent, but wouldn't be true if the region was sufficiently large enough (i.e. extending modestly past the horizon).
We are told that the forest surrounding the mansion was plunged into deep shadow. We're also told that "Solus lost all its light", but I interpret that part differently than you do. Even if we interpret things differently, we shouldn't be disagreeing over the text's literal content.
This is completely false. You can see stars during eclipses, and eclipses only cover relatively small parts of the Earth's surface at a time. It's a bit hard to capture on video, due to clouds, smog, and the brightness of the corona often covering it up, but you can see
in this video around 2:55 a star in the middle-right area of the screen which isn't visible before or after the eclipse. There's also articles
backing this up.
I don't think this works as a proper justification. Textually, it seems like a large amount of the energy used went into the attack on the monsters and healing the girls. For this interpretation to work, the sword would've had to have returned the incoming light to all those positions in the sky between Solus and the planet, and to have only used a small sliver of that to attack the minions and heal those wounds.
So it's weird; you're trying to scale the attack to the full 8 minutes, while also saying that the attack portion only used 1 minute and returned the rest. Without there being anything implying something like this in the text.
And so, I do not find those arguments convincing.