Is ProStructures Object Enabler backwards compatible? Or, do we have to install the specific version of the Object Enabler to match the version of ProSteel/ProStructures?

The latest version of Object Enabler should work fine of any older ProSteel/ProStructures model build in any ProSteel/ProStructures version.

Does ProStructures Object Enabler work with Autodesk – DWG TrueView?

DWG TrueView is not officially supported by the ProStructures Object Enabler. But if the model is saved in the DWF file format then it can be read (without understanding).

Does ProStructures Object Enabler work with Autodesk LT?

AutoCAD LT (light versions) is not valid platform for ProSteel/ProStructures and it is same for Object Enabler.