The ME firmware region needs cleaning after changing the SoC. Also, it seems that the BIOS firmware region is corrupt. I've prepared two SPI images, one with ME cleaned and another with ME cleaned & BIOS/EC replaced with the latest (A17) stock ones from Dell.

How did you flash them? Did you use a programmer? Run MEInfo tool with parameter "-verbose" which can be found at Intel ME System Tools v10 from Intel Management Engine: Drivers, Firmware & System Tools and show us the result.

Well it says Image Failure which makes no sense unless the programmer is not re-writing the SPI flash. Make sure that the programmer is actually reading & writing properly first because the only other alternative I can think of is bad hardware.

The other BIOS also has the same corruption which is strange. Did you use the firmware from the other system? No matter what, the problem is neither at the Engine firmware nor the BIOS because we've cleaned/replaced both. The error Image Failure should point to an inability to read the firmware or firmware corruption. The latter cannot be though so maybe the failure to read the firmware is the result of a faulty SPI chip? I can't think of something else.

Honestly, I don't know what else to suggest. The firmware should be just fine (provided that you are sure both mainboards were the same SKU) so that leaves hardware.

Obviously, the software cannot work if the hardware or firmware is faulty. As I said many times, if we exclude firmware (clean/working ME & stock/working BIOS-EC), only hardware remains. Did you replace the SoC with a known compatible one based on Dell specifications? Did you replace the SPI chip with an identical one? Are you certain that both of these are not faulty?

Then I have nothing else to suggest unfortunately :(