Ok good to know, thanks, will go straight for the hot air then and not bother with jumpers. I don't have reballing equipment so given the small number of connections, was hoping I could get a good enough one-time readout by balling solder manually on either the board or BGA pads with my fine-tipped iron and thin solder (after cleaning up the bottom of the chip carefully with braid of course), concentrating on the flash and power supply pins to make it less likely that inconsistent-size balls on one of the unimportant ones would keep one of the important ones from connecting.
And yeah I checked the battery voltage and it was totally fine at 4.2V, the USB connector was nice and clean too, never had any connection problems before or during. Also tried unplugging all the peripherals (cameras, fingerprint sensor, etc.) from the motherboard in case something was dragging down a power rail. But yes I get the disbelief 100%, because I too have trouble figuring out how an ADB firmware upgrade through the bootloader would immediately make the phone show zero signs of life on reboot, even if something went wrong with the download. Currently trying to convince Teracube support that this should count as a flaw and not "accidental damage" on my part (the difference is a $120 repair fee) as I was correctly following official Murena (e/OS) instructions and there's no way this is supposed to happen if the phone is working correctly, but we'll see how that goes...
For what it's worth, in case anyone trying to make phone decisions runs across this, I wouldn't recommend the Teracube to anyone - low price was nice, but as far as designed-for-repair phones go, the support experience has been pretty awful. The repair guides are half-assed, they keep very few replacement parts actually in stock (they didn't have replacement speakers or vibration motors up until very recently, it took about 2 months for me to get a new USB-C board after I sheared off the mic due to their incomplete screen replacement instructions, and the screen replacement was only needed because the original protective case they shipped with it was far too loose and didn't actually hold the phone in when dropped, which they officially acknowledged at one point), and the whole process has been slow - this time it took about 2 weeks between getting in touch with them and actually shipping out a replacement phone. I get the idea the company is just 10 people in an office somewhere. If I'd had to actually buy a new phone, would've absolutely gone with a Fairphone 4 and never gone back to Teracube, but saving a few hundred $$ is worth the extra trouble I guess.