How to retrieve instance parameters from an uninstantiated (uninserted) family

The trick to be able to read the default values for instance parameters is to get to the FamilyManager.
The family manager is a whole separate area of the API, with separate classes for FamilyType and FamilyParameter. You can think of it as the API version of the Family Types dialog in the Family Editor.
Getting there involves opening the family as a document, something along the lines of:

if (ColFam.IsEditable)
{
   Document familyDoc = mainDocument.EditFamily( ColFam );
   FamilyManager manager = familyDoc.FamilyManager;

  foreach (FamilyParameter fp in manager.Parameters )
  {
      // over simplified, but hopefully you get the idea....
      if (fp.IsInstance)
       {
           string instanceValue = manager.CurrentType.AsString( fp );
        }
  }

   // don‘t forget to close the family when you‘re done.
   familyDoc.Close(false);
}