In other words, in order to nullify one’s acceptance of Shabbos we must weigh all of the affects it will have and determine whether it is beneficial for him.
Based on this I am thinking that it would depend on what day of the omer this occurred. There are seven berachos in the davening on Shabbos. Additionally, there are sheimos that may only be said in a berachah; otherwise they are levatalah as well. In total there are 26 berachos and sheimos levatalah if the acceptance of Shabbos is considered b’taos. If this scenario occurs after the 27th day of the omer, the scale is tilted in favor of considering the acceptance of Shabbos b’taos. Since (according to the way many Achronim explain the Behag) if one does not count sefirah for an entire night and day he has lost the mitzvah, and all of the berachos that he recited until that day are retroactively levatalah, it would be more beneficial for him to nullify his acceptance of Shabbos and remain with fewer berachos levatalah.
Concerning the halacha in this circumstance Reb Moshe rules that (regardless of what day in the omer it is) even after one has davened Ma’ariv, he may count the sefirah of the previous day – provided that it is still daytime.
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