Rosh Hashana: End: Baruch HaMavdil Bein Kodesh L'Chol and Birkat HaMazon

Saying Baruch ha'mavdil bein kodesh l'chol after dark at the end of Rosh Hashana (as for Jewish festivals) does not affect the additions you will then say in birkat ha'mazon.
Situation You washed your hands, said ha'motzi, began eating your meal on Rosh Hashana afternoon, and it is now dark.
What To Do You may say Baruch ha'mavdil bein kodesh l'chol and do melacha, and then continue to eat your meal or say birkat ha'mazon INCLUDING ya'aleh v'yavo and ha'rachaman hu yichadeish alenu et ha'shana ha'zot l'tova v'livracha.