[Android Q&A, Help & Troubleshooting] Tzul:
That doesn't work, unfortunately. Would have made things easier for me while testing TWRP...

I reverse-engineered the two bootloader versions a bit, and found that the Fastboot implementation on this tablet supports only these commands:

getvar ("getvar <varname>" or "getvar all"; shows the value of the specified variable, or lists all of them)

signature ("signature <sigfilename>"; simply prints "command not supported")

flash ("flash <partitionname> <filename>"; there's also "flash partition <filename>" for updating the partition table, but it's not implemented).

More details about "flash" see below.

continue (will leave Fastboot and boot Android normally)

reboot (will reboot the tablet)

reboot-bootloader (will reboot into Fastboot mode)

oem p2u ("oem p2u <on|off>"; "printk to uart" - sends printk output to UART/serial port, neither relevant nor useful to the vast majority of people)

oem reboot-recovery (will reboot into recovery)

oem off-mode-charge ("oem off-mode-charge <0|1>"; enables/disables "off-mode charging", i.e. charging while the tablet is powered down)

If you disable off-mode-charging ("off-mode-charge 0"), then the tablet will boot into Android when you connect it to USB while it is off.

oem append-cmdline ("oem append-cmdline <option>"; Only supported by the new LK; appends option to the Linux kernel command line)

Other Fastboot commands are not supported, not even "official" ones like "boot" and "erase". Commands starting with "oem" are vendor/platform-specific, and not part of the official standard.

More info about "flash":

The bootloader has a sort of built-in partition table, and only the partitions boot, system, cache, and userdata are marked as being "flashable" - that goes for both the old and the new LK. But only the old LK actually checks that info! That's why the new LK allows flashing of all partitions (except for preloader).

However, the maximum transfer/download size appears to be limited to 128 MB, meaning you can't really flash the system and userdata partitions with Fastboot, for they are too large.

Interesting, looks like that's a new update directly to the latest version of Android 5.

I also thought of this: one could actually flash a recovery image into the boot partition and start TWRP that way. Would require some minor adjustment, though. Because the ramdisk in a MediaTek boot image is called "ROOTFS", wherease in a recovery image it's called "RECOVERY". And the bootloader checks that, i.e. it'll fail if the ramdisk in the boot partition is called "RECOVERY" instead of "ROOTFS". But if you change the TWRP image accordingly, you can "convert" it to a boot.img that should work.

Anyway, flash the new LK bootloader ASAP (with dd), that'll allow you to use Fastboot for future flashing at least.

As for the rest: sorry, you're on your own. There might be ways to apply the OTA updates manually after you've rooted/changed your system partition, but that looks like too much work.

Somebody could make a TWRP backup of the latest Android 5 for you... hasn't [MENTION=2895182]clsA[/MENTION] already released one?

Or wait, with the new LK, you can use fastboot instead of SPFT to flash the images contained in my or clsA's releases of the latest Android 5!

fastboot flash boot boot.img
fastboot flash cache cache.img
fastboot flash logo logo.bin
fastboot flash recovery recovery.img (or TWRP)
fastboot flash secro secro.img
[STRIKE]fastboot flash system system.img[/STRIKE]
fastboot flash tee1 trustzone.bin
fastboot flash tee2 trustzone.bin
[STRIKE]fastboot flash userdata userdata.img[/STRIKE]

Otherwise, find a way to get SPFT working, it's simply the best way. Again, there's a Linux version you could try.

Only files with the extension ".img" will show up in TWRP's file list there. The extension is specified by the theme, i.e. it could easily be changed, but unfortunately only a single one is supported, not multiple ones. Easy solution: if you want to flash a .bin file for example, simply rename it to .img in TWRP's built-in file manager.