If the sellers are truly willing to assist you, have them grant you a use and occupancy so that you may move in and occupy for a reasonable fee until they have fully remediated and can provide all required documents. Understand that NJ has recently changed many of its ways it handles these cases. It used to be that DEP handled everything but I believe due to the back log of up to years they may now be allowing contractors who are certified to handle closure of the case. In any event I wouldnt rush to close. (I personally had a $300K clean up)
How much you ask the sellers to concede will depend on the amount of the delay and the inconvenience you are put through.
And I agree, 30 days is quick.
I would suggest to you to continue with the sale. The seller's are being response to you needs, which is very nice of them. 30 days after our closing is quick for a full remediation. Are they moving the oil tank to a new location, or replacing the heating system to gas? That would affect the amount if any reduction is needed. The soil should be in compliance with the NJ guidelines, after the remediation. Good luck!
I would not worry abour closing on the home as long as the seller gives you proof that the leak has been remediated according to the EPA both local and federal. Each locality controls what the consider acceptable remediation. It should be documented by a company which deals with this type of cleanup. Your real estate agent should be able to lead you in the right direction. If you need more information, go to this website.