Does mac OS install process erase flashdrive data DURING the install? It's well-documented that creating an install drive erases the target medium, but what about later, during the install process ?  

i.e. can I create a bootable flash installer, and then add files to it, knowing that a later use of the flash for an installation will not erase those files ?
In my experience you can indeed put files on a flash drive that you made to boot and install macOS. The install process only reads from the flash drive. So your utilities should be safe.