1- if one is unsure as to whether he had said bircat hamazon but did not eat to satiety and therefore doesn’t require to say it m’deorayta and only m’derabanan one should eat again to be able to bless m’deorayta (says the Ben Ish Hai) ; q- what exactly is the process for this? should one wash hands make ‘alnetilat yadayim’ and make hamotze? only make hamotze? just simply eat more bread?
2- similarly if one is unsure to whether he had said a bracha achrona me’eyn shalosh and one prefers to eat more than to hear someone else release him of his obligation how should he eat more? simply eat more an/or make the bracha rishona as well?
Under ordinary circumstances, one can simply continue to eat more, without making a new hamotzi. If a firm decision was made to finish eating (for instance, by saying “let’s bench,” or some other way), it is preferable not to eat more. If mayim acharonim was washed, one should make a new berachah before continuing to eat.
Although in times of Chazal a meal could be ended by removing the table, which obligated a new berachah for continued eating, nowadays a meal continues until birkas hamazon is recited, or until mayim acharonim is washed.
See Biur Halachah 179, s.v. ein.