emmc - mcu supports 4.2/4.3 but device is 5.1
Question #1 - Can hardware that supports 4.2/4.3 be used to read and write sectors from a 5.1 emmc device?


Probably not. The EMMC spec dropped SPI support around the 4.5 version, and most simpler MCUs do not have eMMC in hardware (OP did not tell us which speicifc model will be used). These would require SPI support in order to be able to talk to the eMMC.

SD cards still support the SPI protocol, which is almost always used with MCUs.