Use the DocumentManager to get a list configurations. This is basically the same as using GetModelDoc2 when the component is not suppressed (unless the part open - which I'm assuming is not since it's suppressed so that's good a sign). GetModelDoc2 would return the ModelDoc2 from the model file in its last saved state.
2012 SOLIDWORKS API Help - Configurations Property (IPDMWDocument)
Let me know if this works out for you Jana Stahn !
Thank you for the hint. Unfortunately, it is not ideal for us to use another API (more administration effort) if not definitely necessary.
My colleague gave me a solution that finishes what I started: The trick is to change some SolidWorks settings before opening the component model document file.
IDocumentSpecification copmDocSpec = _swApp.GetOpenDocSpec(comp.GetPathName());
compDocSpec.ReadOnly = true;
compDocSpec.Silent = true;
_swApp.DocumentVisible(false, compDocSpec.DocumentType); // allows opening the file without opening another window
_swApp.SetUserPreferenceToggle((int)swUserPreferenceToggle_e.swLockRecentDocumentsList, true); // prevents the opened file to appear in the recent document list
compDoc = _swApp.OpenDoc7(compDocSpec); // open file - user won't notice
_swApp.SetUserPreferenceToggle((int)swUserPreferenceToggle_e.swLockRecentDocumentsList, false); // change back
_swApp.DocumentVisible(true, compDocSpec.DocumentType); // change back
Now call compDoc.GetConfigurationByName(conf) and check if the result is null.