My guess is that there might be a lot of tools around or you just slightly modify a popular tool like libreoffice. We only need to replace the actual key that was generated from a "fake" password with the key that you now know.
But I can see a lot of tools that might work (not tested myself), like this one: https://github.com/nolze/ms-offcrypto-tool that can also decrypt with the key (by specifying the key with the -k option).
update: it seems this tool only works with AES, not with RC4... but there should be other tools that support older decryption algorithmshttps://github.com/nolze/ms-offcrypto-tool
But I can see a lot of tools that might work (not tested myself), like this one: https://github.com/nolze/ms-offcrypto-tool that can also decrypt with the key (by specifying the key with the -k option).
update: it seems this tool only works with AES, not with RC4... but there should be other tools that support older decryption algorithmshttps://github.com/nolze/ms-offcrypto-tool