Before flashing to internal you could have taken backup from internal eMMC (using ddbr) to restore when anything goes wrong or should you change your mind and decide to revert back.
The install-2018 script is text file and easy to understand. So certainly open source and also self documented.
Nevertheless, I always run it step by step manually (did so twice on different s912 kvim2) as to double check for myself what is being done.
Basically it partitions eMMC and copies SD to eMMC. Then it adjust few text files to refer to eMMC in stead of SD.
Are your 2 S912 boxes equal model? Then maybe you can backup eMMC from one and restore it to other? Just a thought...