I believe the Mojave installer puts the system firmware updater onto the internal drive. I am not sure about macOS 10.13, but with macOS 10.14 Mojave I can say without a doubt from personal experience that the Mojave installer will quit with an error about 'EFI firmware error' if the laptop does not have an internal drive installed with a valid GUID partition formatted as MacOS Extended (Journaled). HWTech has probably seen this and can opine on whether that will not work. then you are proceeding one step at a times and can fix any issues as they occur. when all is working well, you can try moving the new fully-updated SSD inside. I think you should be able to use an external drive to install 10.13.6 and get the firmware installed, then move easily to an upgraded SSD. The Mac that has been very fussy about firmware and the genuine Apple Internal SSD was the Mac Pro (desktop) 2013 model. Maybe an independent Apple Authorized Service Provider would have a spare drive SSD and would be willing to install macOS 10.13+ for you so the system firmware is up to date so yo can use the OWC Aura Pro X2 SSD.

If the laptop already had macOS 10.13+ installed, then you may not have any problems. It might be possible to use this setup to install macOS 10.13+, but there are no guarantees. Remember this option is very risky as it is completely unsupported by Apple and even the third party SSD vendors. It might be possible to use a standard M.2 SATA SSD with an appropriate adapter made for a Mac. I know from personal experience that the macOS 10.14 installer has this requirement. If the macOS 10.13 installer installs a system firmware update, it will require a properly partitioned and formatted internal SSD. If your laptop has never had macOS 10.13+ installed, then the ONLY way is to temporarily install an original working Apple SSD so you can install macOS 10.13+. How can I update my MacBook with the dead SSD so I can use the newer 10.13+ SSDs? I would like to use a newer OS X, and I know my macbook is compatible (e.g., with Mojave).

Owc aura pro ssd guid partition table