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I'm not sure how to do what you want with customizing Mint directly, but a possibly simpler alternative solution is to just send two clearly distinguishable USB drives (e.g. label them "1" and "2" with a label maker or get two drives with very different colors) and tell him to install (unmodified) Mint from the first and then have him run a program you provide on the second after that's done to make the other changes.
I have a folding card table that currently serves as my desk. I don't know how old it is -- 1960s, maybe, based on the style of the brand/sales label on the back? It's almost certainly older than I am, at least... Got it from my uncle back when I was in college and its still working well enough that I haven't bothered to replace it.
I went back through Wikipedia's current events from June 2025 and put together a rough timeline: