Does DOL say the reason for Denial if they deny any case?
YES. When DOL (or INS) deny case, they send a detail letter justifying all the reason(s). You or your attorney might not agree but that's a different issue.
Also once they deny, the case converted to NON RIR. Can company apply for 7th year extension since the labor is still pending under NON RIR?
If DOL denied you RIR application, they are asking you to convert RIR case to N-RIR and you agreed, company can apply for 7th year extension based on that provided there are no further issues.