Although it is not documented, it is in fact possible, and even reasonable, to use WebView2 in MinGW. Once you have the necessary files from the NuGet package, it's simply a matter of using the CINTERFACE variants of the COM functions.
I don't know what the best practices are for implementing COM classes in C so I came up with my own approach for the event handlers. It should generally be good enough for this use case, although there is a lot of room for improvement. The point of this demo is more to show that 1. it is possible to do this 2. how one might do it from a technical sense. Hopefully, actual implementations will be better than my weeknight hack. :)
Simply compile with a MinGW toolchain. Msys2 + MinGW64 should work fine. Run make:
make
and if it works, you will have webviewdemo.exe in your working directory.