I apologize for the delay.
Please find attached a document about how to boot the i.MX8MM through tftp/nfs.
For your second problem.
The way you are using it is the best way.
Regarding your Android questions.
For the first question, is possible, but kind of difficult to make this, since you will need to modify the UUU script. What I recommend you to do is download the desired images through fastboot.
To make this please follow the below guide:
Before using fastboot, Android SDK must be installed on the host, and the target board must boot up to bootloader. Also, U-Boot must be downloaded to the MMC/SD device with all the partitions created and formatted.
You can achieve it doing this:
1.- Run "reboot bootloader" in console after boot, the device will reset to fastboot mode.
2.- In the host side, enter the Android SDK tools directory and find the fastboot utility (fastboot.exe on the Windows OS, fastboot on the Linux OS).
3.- Copy the images that will be flashed into the images "folder".
4.- Then you will be able to flash the image with the following commands:
$ fastboot flash gpt images\partition-table.img
$ fastboot flash bootloader images\u-boot.imx
$ fastboot flash boot images\boot.img
$ fastboot flash system images\system.img
$ fastboot flash recovery images\recovery.img
$ fastboot flash vendor images\vendor.img
5.-After flashing the images reboot the device with the following command
$ fastboot reboot
As a side comment, fastboot does not support flashing the bootloader into the NAND storage.
Hope this information can be useful for you.
Best regards,
Diego.