FYI, I've made some progress in my attempt to parse the JS file to a format I can use in C#. I found a library that can convert JS to JSON, which means I can parse the JSON, which I have a lot more experience with. Progress is very slow, but I do think it's possible now.
The next step is to list the minimal parts of a class that are necessary on my side to combine with the PDF data. Once I've created that list, I'll know if it's actually doable. I've collected a few sample JS class files to make sure my system will work with most JS files out there.