Hi guys!
The programmer volunteers among you (and future modders) are gonna love this one.
As of SVN revision 6197 and a rebuild of the DLLs, attaching to Unity with the official Visual Studio Tools for Unity and debugging external DLLs such as AIWarExternalCode.dll just works(TM).
Thanks to the
Kerbal Space Program modding community for putting me on the right track! The key was to use a proper pdb2mdb.exe that's compatible with Roslyn-compiled dlls. The one that comes with unity 2019.1 is ironically not compatible.
To debug you can just load the AIW2Unity project in the editor with the ActualGame.unity scene, attach VS Tools For Unity, set breakpoints where you want, and press play.
If Chris adds to Google Drive a standalone build with the "Copy PDB files", "Development Build" and "Script Debugging" options enabled, it would allow people who don't have unity installed to attach a debugger. Perhaps there's a way to add such a build to Steam too, perhaps a release one with only the Script Debugging option enabled? Would save modders who want to debug their AIWarExternalCode.dll the trouble of replacing some files to produce a debuggable build.