can chi'n replace the "ti'n"here ?
and why isnt there dych, or in this case "wyt"?
The wyt/dych is omitted here because it is just how we would say it. And yes chi would be fine here.
<dych chi'n iawn> was corrected to <Dach chi'n iawn> ?? What's the difference between dych and dach?
Dych is southern and dach is northern, they are interchangeable and should both be accepted.