PBP disappointment

[bcnaz]

1)  It would probably be an excellent idea to have a step by step guide in the newbie section of the forum.

2) I have not tried Manjaro on the Pinebook Pro, I started following Pine64 in September of 2019, and I ordered one immediately.

My first and second Pinebook Pro's came with the mrfixit's version of Debian, that was fairly simple to use and while I did have a few minor issues,  with the help of this forum I was able to resolve all the problems.

I still am using the mrfixit version of Debian, but I did recently test drive the latest release from Debian's website of the 'arm64'  on an sd card and it did run ok, even the wifi worked.

I have also tested the mrfixit/Debian (from the Pine wiki)  flashed to an sd card, and that does work for me.

When you say "etched" I assume you mean 'Balena Etcher' .?

You said you have the usb/eMMC  adapter ? 
 You should be able to use that adapter to flash a new operating system to your eMMC .
     ( There are a few  'ready to use'  operating systems in the wiki. )

 NOTE :  It is a different download for the sd card and for the eMMC.!  !

ALSO :  I just re-read your original post, you do NOT need to remove or turn-off your 
           eMMC to boot from the sd card..!
                 It will always boot first from the sd card "IF"  one is in the slot.

Good Luck