Intel Arc will not support DX9

Yeah, that would be great, but I couldn't find any direct mentions of that and I don't have the required knowledge to dive deeper.

 

Your 3 points sound good.
About 2) from my understanding https://github.com/microsoft/D3D9On12
Dxvk replaces d3d9 DLLs and therefore can be faster as it can skip validation logic normally done in d3d9.
D3d9on12 sits behind the classic d3d runtime and as you say is where typically the native driver interface would sit. So regular d3d runtime and its cost still apply.

So that gives us:
App > d3d9 runtime > driver interface (DDI). This is 9on12 > d3d 12 runtime > hw d3d driver

For dxvk it's a bit shorter:
App > dxvk replacing d3d9 runtime> hw VK driver