Question

1. Dad is in back of the door. [O]
2. Dad is behind the door. [O]
3. Dad is at the back of the door. [?]
4. Dad is in front of the door. [O]
5. Dad is before the door. [O]
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I'd like to ask you the question one more time.
Some native speakers seem to use #3 and some don't think #3 is correct.
Does #3 mean #1 or #2? Or is #3 totally different from #1?
Would you clear that one more time. If a student use #3 to mean #1, is it grammatical?

6. I hit the chair in front of me. [O]
7. I hit the chair in back of me. [O]
8. I hit the chair at the back of me. [?]
9. I hit the chair in the back of me. [X]
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Is #8 not the same as #7?

Answer 1

I don't know any native English speakers who would use 3, 5, 8, or 9.

1 and 2 are clear, and there doesn't need to be any substitution. Same with 6 and 7.