Replying to my own post because I've found out what's going on.
"Currently, Get(TemporaryPath) is returnung "/System 250/private/var/folders/4D/4DANsW03HG0DdU1dyriORE+++TI/Cleanup At Startup/S10/"
"/Cleanup At Startup/S10" does not exist on my drive."
Although the function is not returning what's expected, the folder "Cleanup At Startup" exists as an invisible folder nested into "var" which is itself an invisible folder, and FMPA creates "S<n>" when needed. It's still odd that the TemporaryPath is not what it should be, and that although I can use "Go to folder" to find this directory, a shell script insists it doesn't exist.
I'm going back to using the Desktop as my (scripted) temporary path and scripting the deletion of the file as soon as it's sent to the printer.