Yes, keeping SinoVoip image on NAND while running OpenWrt from eMMC is not supported. You can do it the other way around though (OpenWrt on SPI-NAND, SinoVoip stock on eMMC) and shouldn't experience any problems.
That's odd, accessing NAND in OpenWrt-built U-Boot on the R3 mini works fine for me, as does Ethernet support in U-Boot if you have the Airoha EN8811H firmware loaded on the flash or eMMC.