1. Sir, to make the compliance in respect of employees even engaged for maintenance or extension etc. of the buildings/premises of the covered factory/unit is the liability of the principal employer. When the so called construction work/maintenance work was started, your unit should have called for necessary details of employment of construction workers engaged by said contractor and should have asked the contractor to make the compliace or in fail should have made compliance by the principal employer. Even now (if assessment has not been finalised under section 45A) you can call for the details from your said contractor and pay the contribution on actual amount of wages under said Act.
2. If compliance is not made and no bifurcated details are furnished to the said department, I feel that the said department will have no alternative except to claim contribution on total amounts as mentioned by you in your thread treating as if all such amounts were in actual coverable wages.