Docooler M9S Pro s905x 2G/16G p212

I have previously had a few of these boxes ranging from Android v5 through v6.0.1. Being generally satisfied with the product, I recently bought two more of these boxes which should have arrived with Android 6.0.1. As it turned out, they were flashed with a very bad version of 7.1.2.

What is working:

• Wifi 2.4
• Bluetooth devices seem to connect
• IR remote that arrived with the box
• USB mouse

What is NOT working:

• HDMI Resolution is 720p, even though the MBOX launcher settings state that the box is running at 1080p. I tried switching between different resolutions without any effect.
• MX3 remote: only the air mouse works, none of the buttons, nor the keyboard work
• USB keyboard not working
• Bluetooth keyboard connects but typing has no effect

Other firmware issues:

• SuperSU: At first boot, you will receive a notification: “The SU binary needs to be updated”. If you try to update the binary the box will get soft-bricked.
• AMLogic firmware and USB Burning Tool: I tried so many different firmware images without luck. This one is “"working"” (v7.1.2 20171025): https://mega.nz/#!6N4TwKAI!7Gt28mgv5...NgJ2i2a813ja5U. It flashes the box, but has all the issues mentioned in this post. I did not manage to find a working 6.0.1 image to downgrade with.
• TWRP: Magendanz’s twrp.p21x-3.1.1-0.img, posted at “TWRP for Amlogic devices” at 07-26-2017 17:58, works well – both Backup and Restore work. I was lucky that I had two of these boxes, so that I could use TWRP to backup and flash the one that got soft-bricked after the SuperSU lock-up. Download link: https://1drv.ms/u/s!AoqocX31ecnXgZx-RKW1MkpzTHfiWA.
• LibreELEC: I did not manage to find a working device tree (dtb.img file) so I never managed to boot LibreELEC off the flash card.
• Display resolution, keyboard / MX3 remote and LibreELEC are deal-breakers for me so I did not bother testing any other potentially broken features.

Request:
I do not have any experience with firmware development, but I am willing to try to unpack, modify and repack the provided firmware to make it function properly. All the help I can get, regarding the firmware and generating/extracting the device tree, would be deeply appreciated.
I would also be more than happy if anyone could provide a generic version of p212 TWRP firmware backup with all the above issues being fixed.

Thanks!