NC_OBJECT | ND_TRANSFORM should be sent when an existing object was transformed, and NC_SCENE | ND_FRAME when the current frame or frame data changed. So both are rather arbitrary choices (although they will probably trigger some of the necessary redraws/refreshes).

We need to notify that the visible contents of the scene or more precisely of the view-layer changed. But in addition, selection and the active object were changed, so we should notify about this too. Best copy one of the existing functions adding objects, e.g. object_add_named_exec() does this:

WM_event_add_notifier(C, NC_SCENE | ND_OB_SELECT, scene);
WM_event_add_notifier(C, NC_SCENE | ND_OB_ACTIVE, scene);
WM_event_add_notifier(C, NC_SCENE | ND_LAYER_CONTENT, scene);
ED_outliner_select_sync_from_object_tag(C);

Lastly, I'm not sure what the best location for this is. If the import happens through the other importer_main(), the notifiers will be missing. Should that function be used for anything else than the tests, that would be an issue.
I think it's reasonable to say that these functions only deal with the scene data, while the UI should be handled by the operator, or whatever calls this. We do this in other places too. That means the snipped above is added to wm_obj_import_exec(). The OBJ functions should document that they change selection, the active object and that thus the caller should trigger UI updates if needed.

So my suggestion:

• Paste snippet above into wm_obj_import_exec()
• Note in the function comment of at least OBJ_import() that besides adding objects, this changes selection and the active object, and thus the UI needs to be updated accordingly.