Yes. You can use the Import task:
<Import Project="PathToMyIncludeFile\Include.proj" />
And yes you can create a batch file to run msbuild. The syntax is
msbuild <project> /t:target[;target] /p:propertyname=propertyvalue
Where the targets are defined in the msbuild file and the properties are any properties defined in the file. If you don't specify a target, the default that is defined in the msbuild file element will be run. Here are a couple of examples:
So to run your build with the Clean, and Compile targets:
msbuild myproject.proj /t:Clean;Compile
Or to run your build with a target of Compile and a release configuration:
msbuild myproject.proj /t:Compile /p:Configuration=Release
Or to run your build with the default target and a set a version property:
msbuild myproject.proj /p:Version=2.0.0.1
Command line parameters take precedence over the values defined in the file. So in the example above if you had the version defined in the file as:
<PropertyGroup>
<Version>1.0.0.0</Version>
<PropertyGroup>
The build would run with a configured version of 2.0.0.1
As usual, check out MSDN for more info.