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

[rotwang]

This sounds like two separate issues combined into one.

First thing is that yes, I think that U-Boot flash completely toasted the U-Boot install. With the version that comes with Manjaro, the power LED should turn yellow/orange as soon as U-Boot starts executing. If the LED stays red, it's not running at all. The big problem with this is that most of the boot options we're familiar with are reliant on U-Boot. The RK3399 SoC by itself will try to boot from SPI, eMMC, and then SD in that order. It will not attempt to boot a USB drive.

That leads to the second problem: SD cards. The PBP is pretty picky about how images are flashed to an SD card. Make sure you just flash the .img file (not the .gz or .xz it may be compressed in) to the card. I'd recommend using Etcher or Win32DiskImager on Windows or dd on Linux to do the flashing. If that doesn't work, try other SD cards. If that doesn't work, given that your PBP got repaired, I'd recommend opening up your PBP and checking if the two boards have their ribbon cable between them connected correctly.

If all the above fails... contact support again.

Thanks for the helpful set of tips! But based on other forum posts on this topic, I had already tried all your SD card recommendations before I contacted support the first time. (I know I posted a long post, but it's all in there.)

Thanks for the note on USB not being an option. That leaves me with aimlessly kicking the SD card around (trying various cards, images, re-seating the cable to the mini-board again) or buying the eMMC->USB thingy.