half a dozen of people should be half a dozen people. è No, “of “ should not be there.
without too much effort from my side. on my side or from my side? not sure, but definitely on my part. è I see, you are determined to change my vocabulary with yours.
So, I smiled understandably and remained silent. understandably or understandingly? è Which one? They have different meanings, but both fit into the story just fine.
so I started shaking my head horizontally, horizontally? If it's vertically, it's nodding!
I knew I have stretched it a little bit too far. I knew I had è This is a good pick.
is this dentist’s office, I feel the punch line should be "Is this the dentist's office?" not 100% sure though. è I know you are quite obsessed with “a”, “an”, “the”. However they are totally dependent on a writer’s intention, not grammatically right or wrong principles as you may think.