I believe the halacha regarding someone who mistakenly started to say shmona esrei for Shabbos during the week and remembered before רצה is that he can continue with atah chonen and it is not considered a hefsek. Does anyone have any sources on this similar question of mine regarding רצה in ברכת המזון?
edit - Gershon Gold gave a nice answer quoting the Ktzos Hashulchan, however I am still looking for sources that argue.
