as a softdevice you just program it into the nrf chip softdevice section there is no need to change nrf_nvmc_page_erase() is a program which you write into the chip program section but not softdevice . Not sure if you now that nrf52x have three sections dfu/application/softdevice in sdk ,softdevice cannot be modified it is in hex file not sure what you are asking about.
It is not only not recommended. nrf_nvmc_page_erase() will not work if a SoftDevice is enabled. The NVMC is protected by the SoftDevice so that it can schedule flash operations, so you need to use the SoftDevice API for that. The corresponding SD API is sd_flash_page_erase(). And them you also use sd_flash_write() for writing.
