Is there a lesson that introduces the "na" negation in general?
(And is there a reason why "Beth os fyddan nhw ddim yn iawn?" is incorrect?)
Your suggestion is perfectly correct and was not added, that's fixed now, thanks for the comment.
Na fyddan nhw = Fyddan nhw ddim, as far as I can see, some better linguist may correct me here though.
os na fyddan nhw... is Cymraeg da. More informally, os fyddan nhw ddim... is also used.