mythological unicorns are typically imbued with supernatural grace. it could be a latent ability of unicorns to sense and instinctively avoid clumsy mistakes with their horn. like cats do with their whiskers, and we all know ponies are basically cats.
also, mythological alicorn is typically made of straight bone and not kerain. even then, it's more like tooth than skeletal bone, you would expect it to be exceptionally resistant to breakage but if it breaks then you're fucked, it's over.
that doesn't really go with the doorstop-spring gag but whatever it was just a gag.
as far as rounding it off, i'm guessing you don't remember that basically every powerful spell that's been animated involves a magical power focus at the tip of the horn. assuming that's significant then rounding off the horn could be the same as removing it entirely.