My pbp is a sleek, black, flat brick, and won't boot from any kind of media

[rotwang]

No boot SOLVED
My experience for what it's worth:
I went through all the suggested fixes (described in a previous post: flashing zeros to SPI, checking daughterboard cable, etc) and finally started over. I had tried so many approaches, that I was losing track. I used a second computer, a 2gb HP celeron laptop running puppy Easy OS on a usb stick.
I disabled the 64gb emmc and removed it. I had already flashed the SPI with zeros. That left only the sd slot for booting.
I downloaded balena-Etcher on the HP, unzipped it, and changed permissions to allow execution. I downloaded the Manjaro ARM KDE plasma... 21.06 . I put a 32gb Kingston HC1 "Canvas" sd into a usb multipurpose reader from IOGear Model GRR304SD and put  reader into a USB 2.0 port. I used Etcher to flash the 21.06 file onto the sd card.  After the file was flashed (5 min), Etcher ran a validation check which failed. I redid the flashing, and the validation check said the flashed image was fine.
So, I loaded the Kingston sd card into the bare bones PBP, held the power switch down for a minute, clicked the tiny reset button on the motherboard, pushed the power switch for a few seconds, AND, for the first time in two weeks, the PBP lit up and went into action.
One down, one to go.
I removed the sd card, enabled the emmc ,  pushed the power switch and got nothing. So:
I loaded the emmc card on to an Ordroid USBEEMMC (Olympian LED) reader that I bought for the purpose, and put it into the 3.0 USB port on the HP. When mounted, I looked at some of the files and found that I had previously flashed the Manjaro KDE plasma...21.07 file, the one meant for intel machines.  Using Etcher again, I flashed the  ARM 21.06 file that  worked on the sd card.
I reinserted the emmc card in the PBP, and it worked fine.  Put the sd card in as well; and that worked.
So, it was all my stupidity. I highly recommend Etcher, the Ordroid USB-Emmc reader, and the IOgear USB reader. I used the pricier sd cards to be safe. If I have trouble in the future, I would not flash zeros to the SPI  until every other option failed. It took me days to do and I'm not sure that it was needed.
Loving my PBP.

Your experience leaves me with the frustratingly remote possibility that the problem _is_ software and not something physically wrong with the sdcard reader. I am not super excited to go out and buy multiple high-quality SD cards, a usb connector for flashing the SPI, and a eMMC-to-USB reader, and then spend weeks more futzing around with this thing only to find out the sd card reader is physically not working anyway.

And it just seems much more likely that (in my case) the sd card reader is not working because Manjaro wasn't detecting the sd card the one time it booted after the keyboard was replaced. (That's the same sd card and arch linux image that Manjardo _did_ detect and boot from _before_ the keyboard was replaced.)

Given all that, wouldn't y'all say that the most likely explanation of what happened to my machine is the sd card reader got hosed when support replaced the keyboard, and that was followed by me hosing the eMMC with a uboot flash and Manjaro update? Leaving me with no way to boot my machine.

Should I go back to PINE64 support now? Or do you think I should get some cables and start messing around with flashing the SPI? 

Thanks for all the ideas! I do like trying these things, but on the other hand I've had this PBP for like two months now and it has booted exactly twice. I look forward to the day where I'm wrestling with Arch linux's idiosyncrasies instead of the PBP's.