Is it just me that really wants to FOIA the firmware code to read it?

What is it written in? Assembler, surely?

What is the development toolchain like in 2023 for a 70s computer? Do they have an emulator? What happened to the spare development Voyager they had lying around?

Not Voyager but here’s code from the Apollo program: https://github.com/chrislgarry/Apollo-11/