Depending on your board revision there may be a problem with smbus pins for the NVMe having the wrong voltage level (3.3V instead of 1.8V). The easiest fix is to mask those pins using a bit in plastic tape (SMbus is not needed for the NVMe to work), or to remove R228 and R230, should you be able to locate them.
See also
The easiest and best way to do this is using the bootloader menu on the serial console.
As micro SD and eMMC cannot be accessed at the same time, you will need to first select the item to flash to SPI-NAND, then boot from SPI-NAND and install to eMMC.
Just use select the appropriate menu items in the bootloader menu.
Instructions on BPi Wiki unfortunately apply only to BPi firmware, not for official OpenWrt.
With enabled hardware flow offloading you should be able to achieve 10 GBit/s wirespeed with PPPoE. I haven't tried this though, lacking a fast enough PPPoE server. Let me know your results, and check /sys/kernel/debug/ppe0/entries and /sys/kernel/debug/ppe0/bind to be sure that hw flow offloading is used correctly.
If the connector is USB Type-C then yes. Best would be you provide a link or photo for that specific serial adapter, I can't find it.