Well you have the general concept nailed down for the most part, I think. It's less of a theory than it is what you're supposed to find out. A few things to mention: Yorihime confronting the vampire party not only isn't aggressive, it's the invaders that would be the aggressors, not Yorihime. On top of that, Yorihime was only there to deal with them because the Watatsuki sisters knew the vampire party were just the distraction for Yukari and that Reimu would be with them. Nothing about that encounter was all that dangerous.
Second, the Earth is not a threat to Lunarian society whatsoever. While there haven't been any solid conflicts between the Earth and the Moon besides maybe the first Genso-Lunar war, even the threat of impurity isn't huge. The Lunarians don't want impurity in the lunar capital, but it isn't as if a little impurity is a grave danger on its own. Earth-dwellers aren't going to be showing up in the capital en masse and dying all over. Note that they kept Reimu there for like a month and it wasn't a big deal. Also, even if hypothetically the lunar capital did gather impurity and disease entered, the Lunarians have arbitrarily good technology and could probably cure any disease that shows up. It's more the principle of there being impurity at all.
So this isn't really the reason they would be okay with wiping out stuff on the Earth. Really, the Lunarians for the most part have a nonchalant and pitying attitude towards the Earth. They view it as pathetic. Toyohime atomizing the area is more like kicking an anthill. The anthill isn't a threat whatsoever, but you don't care about the anthill, you view the ants as meaningless creatures, and it would be easy to just kick it. Toyohime eventually choosing not to wipe the area is just the same as going "well, they're just ants, so whatever".