Can I say "Iawn dych chi?" ?
You could, but it would sound really weird.
Normally you would either say 'Iawn?' on its own or:-
dych chi'n iawn? = (are you) OK? (polite or plural)
wyt ti'n iawn?= (are you) OK? (informal or to a child)
In what context can 'Iawn' be used?
Obviously there's "Da Iawn" to say 'very good'. But can you use it to reply to a command? e.g. "Wyt ti'n gallu car'u siop, plis?"