Since it's been a while since this has updated, people might need a reminder.
Luka, is currently possessed by an entity calling themselves Amy (Ch 76), when this happened is currently unclear. When she was obliquely introduced, the Possessed Assistant Guild Master noted that they had changed bodies (Ch 37 p12). So, Amy could have been prepping this spell using other bodies, for years, before possessing Luka.
Considering that it took years for Amy, to write the spell on the City Walls, it's not a simple matter to just do a precheck now. It's a last resort, that could still backfire. Trying to do that while the Guild is after her, would take literal hours.
And Tohru, is essentially isekaied to be the Apostle of the Goddess of Fate/Plot Armor. So, the Goddess could essentially make anything a heavy handed coincidence. The Goddess could have noticed the spell, and had Tohru assigned a room where that arrow was guaranteed to disrupt the spell, or she could have directed the arrow itself. But, if you disregard direct divine interference, people are ignoring the fact that the sword/arrow itself has weird properties. It went through walls like nothing, it ignores normal humans. So it's essentially a laser beam, until it hits something with specific qualities. So, it's less that the arrow was specifically shot at the spell, it's that the arrow itself can't be stopped /except/ by the spell, and/or specific entities. So he could have shot the arrow from /anywhere/ inside Finris, and it would inevitably interrupt the spell.