I'm not really sure what you're complaining about here. .net core is split into tiny packages - if that is hard to handle in your very special environment, you get to use special solutions to make it work.
That being said, WinForms is also "just" a Win32 wrapper, I don't see a compelling reason why a similar wrapper wouldn't be possible in pretty much any language. .NET 4.6 is a fine choice too, especially because you're not forced to ship runtime and standard library, as Windows already has both.
We don't even require the internet for building/development. In .net land you require internet to get the dependency you're using the first time. If you want to get them all and distribute/install before you start development, you can totally do that. It's just not the default behaviour.
Contrast this with e.g. Zig for Windows is a ~60 MB zip file which contains the entire Zig/C/C++ compiler with cross-compilation support for basically everything, and also has complete Win32 headers + libraries in it. There's even DDK headers in there, though I'd expect to do some legwork to build drivers for NT with Zig.