This line:

<ReferencePath Include="$(TargetPlatformSdkPath)UnionMetadata\10.0.26100.0\Windows.winmd" />

is necessary in the various UAP targets, but it's a bit of a hack, and seems to be fragile in the face of build agent changes. (We'd rather be using an older version, but we had to bump from 19041 to 26100 after a build agent change.)

Is it possible to use something like this instead?

https://github.com/microsoft/CsWinRT/blob/master/docs/embedded.md

Or perhaps there's a NuGet way to obtain this metadata instead?