" If the shooters had not had firearms as their fallback plan..."
Actually, they didn't.
There was no fallback plan.
The guns were there for the sole purpose of picking-off survivors fleeing the successful bombing that never happened.
Indeed, if dynamite were as available to the Columbine shooters as it was in 1971, then their credit cards would've bought them something almost foolproof off the shelf. As it was, their failure meant that they had to resort to what WAS almost foolproof off the shelf.
"They have not forgotten that socialists disarmed their victims before committing genocide in Germany, the Soviet Union, China, and other nations."
When the invading force arrives from Finland or from the hills in the Chinese hinterlands with enough arms to defeat the entire national armed forces; then yes, removing guns from civilians in the regions they occupy is the natural next step. But it seems to me that you are ignoring the obvious first step.
And in the case of the Austrian, having coopted the entire army, navy, air-force and police; he didn't really seem intimidated by the difficulty of disarming the population.