... No? That's why I said "if"? .NET Core can compile for Linux. You don't need to know any specific codebase to know that.
That'll do it. I didn't realize parts were written in C++; getting runtime-less languages to be cross-platform is quite a bit more difficult than something like .NET.
No, but you must be, lol.
WinForms and WPF have .NET Core implementations, from what I understand. Direct2D and DirectWrite are pretty impossible to get around, though. You'd need to change the whole graphics engine out for something based on Vulkan or OpenGL - which I'd bet almost anything won't be even close to a drop-in replacement.
And I don't really know anything about WIC.
Man, I hate having to use WINE myself. Steam's Proton is great, but managing a WINE environment myself is no fun at all. I always feel like it's not _quite_ right.