We have very strict rules on what can be installed and there is no way around them and legal repercussions it I could. I can't even install PIP, let alone run it.
If we accept that, is there any way to convert an Excel .xlsx file to csv?
Is there a DOS command line that I can run? DuckDuckGo doesn't seem to think so.
Is there a Python module that I can simply download, with having to use PIP? (preferably a single file, rather than a directory tree, as I will have to distribute it with my script))
Is there any other way?
See my script tabxlsx.py - single file without addtional dependencies.
Installation can be done by copy-n-paste to a target system. It uses the internal xml and zip modules from standard Python to read and write Excel xlsx files converting to/from other formats including csv.