It is 'bwytewch' not 'bwytwch'. Look in Y LLyfr Berfau under 'bwyta'.
Bwytewch appears to be a literary form, bwytwch the colloquial form.
'Rwan' is definitely a northern welsh word! Growing up speaking welsh in school, I know this to be 'nawr'. I don't know which is the most common though?