d_m

In the following thread I essentially did all the steps I had mentioned above.  Proceeding slowly and carefully, I felt confident to flash the SPI, once I had determined that indeed it was empty.
 
In the end you were right, the key is putting tow-boot on the SPI.  I mean, that is certainly the easiest way I think.  And the way I did it, I did not even have to open the case (which I was getting very tired of doing by this point, LOL).  I have got Armbian working now both via SD card (the other day) and/or eMMC (just tonight).  Detailed instructions can be found here:
 
https://forum.armbian.com/topic/27598-getting-armbian-working-on-second-batch-mid-2022-pinebook-pro/

Honestly I would not have flashed Tow-Boot into the SPI except that at some point a U-boot update made my PBP unbootable. So I had no choice.

I had to actually open the case to do this (and it took a few tries to get everything working) so it's certainly not for the faint of heart.
 
I really wish Pine64 had shipped these things with a more reasonable boot loader from the start.

Looks like you asked awhile ago, hopefully this is still relevant.
 
I've been successfully running Armbian off of EMMC on my Pinebook Pro. Before installing Armbian I actually flashed Tow-boot onto the PBP's SPI (replacing the built-in boot loader). Then I was able to run the Armbian installer and installed directly to the EMMC.

This set up seems to work fine for me. Hope this helps, good luck!