Why can hvis not be used instead of om?
How come this uses "om du ikke hører" instead of "om du ikke kan høre"
"om du ikke kan høre" is correct and so is the other option.
Is it common to say "ei" instead of "ikke"? Is that a dialect?
I guess it's dialect. Switch out "not" or "neither" with "ei" and you got it. "Det er ei en hest eller et tre." You can often find "ei" used for "ikke" in hymns and such.
It is not a dialect word as such, it is perfectly correct Bokmål. It was used more before. A famous example is the opening of Peer Gynt, a play by Ibsen: