According to the Ran, the osek bemitzvah exemption does not apply in the Easy Case but does apply both in the Impossible Case and the Difficult Case. The Ran cites several proofs for the application of the osek bamitzvah exemption in the Difficult Case as follows: In Talmud times, a bridegroom was exempt from reciting Kriyat Shema on account of his preoccupation with his wedding night. Similarly, cites the Ran, a person performing the mitzvah of guarding the unburied dead or digging them a grave is exempt from all mizvot. In all these cases it is possible, though difficult, to recite Kriyat Shema while preoccupied with the first mitzvah. Yet, says the Ran, the exemption applies. In explaining the reason for the application of the osek bemitzvah exemption in the Impossible Case and the Difficult Case as opposed to the Easy Case, the Ran points out that the Talmudic dictum is “ha’osek bemitzvah” (one who is busy with a mitzvah) and not hamekayem mitzvah (one who is merely fulfilling the mitzvah).
The halacha, as expressed by the Rema, adopts the approach of the Ran and this would seem to accord with the position taken by the Rambam in the case of the shomer aveidah.