arhi

looks like a dead bloody card ... you write to it, it ignores writes ?!?!?!?! have not had this before .. wow .. well you learn something new every day thanks for help I'd probbly never put the serial on my own
 

hmm I don't like videos, but yeah you don't have much other possibilities to show what's going on... 
 
First a USB-UART would help yo in case something is visible on uart but not on hdmi, second starting Kernel is normally the last message you get from u-boot before the kernel controls output.. So it seems that he is only shortly able to post something on your HDMI before it f*cks up (maybe he talks a bit more to you on console, might be interesting... )..  But a good thing.. boot.scr (that's where your bootscript lives, which gives u-boot the possibility to set bootargs for the kernel) seems to be loaded and executed properly.. so you can set 'debug commands'  for the kernel.. So there's still hope..  but I think you should record when possible on serial console rather than HDMI and hope that the kernel isn't that shy to talk to you..   
cp boot.scr boot.scr.orig edit boot.cmd with the arguments you want to send to the kernel from u-boot (sitting in bootargs=blablabla) mkimage -C none -A arm -T script -d /boot/boot.cmd /boot/boot.scr Cause you consider starting from plain again.. there's not much you can lose...    
Edit: your u-boot talks much more to you that the one I'm currently working with.. Makes things easier..   

For Allwinner/sunxi devices mainline status can be found here: https://linux-sunxi.org/Linux_mainlining_effort#Status_Matrix
 
Difference between legacy 3.4.x and mainline 4.x for H3 devices is not stated in Armbian documentation because we don't provide official/supported mainline images for H3 devices yet.