I would say "disparity of force" won't be an issue here. A young, reasonably fit person armed with an impact weapon generally meets the legal definition of having a "deadly weapon".
Shop owner, almost certainly a decade or three older than the attacker?? Possibly not in top physical shape? Using a firearm would almost certainly be considered justifiable, to prevent an attack.
Store owner grabbed a broadhead arrow, raised and let it fly. Went through the robber, through a screen door, and stuck in a parked car sitting out front. Police found the robber in an alley a block away after he bled out.
Contrary to generations of Hollywood and TV, that's what arrows do. GO zipping right through "soft stuff" and only stop when they hit something hard (wood, bone, armor plate, pocketwatch, etc.) pretty rare in the real world to find some unarmored trooper shot with an arrow with a couple feet of arrow sticking out of him. Arrows might punch through medieval armor plate, but will seldom punch through both back and breastplate.
And chainmail hasn't been much use against arrows since the invention of the bodkin point.
