Good job getting the tank out before closing!!
Best of luck to you,
My advice is do not close until the property is clean and documented. Period.
Yes, this can be a long process. The problem is that you sold your house and you need a place to move into...I believe that the grounds can be cleaned, it will cost money, but the seller is paying for it.. After the cleanup they will have to take further samples to make sure it is OK.
The biggest wait can be getting an approval from the DEP..
I believe that if the seller is willing to put money in escrow, you should be OK.
Find out from the contractor how much it will cost for the clean up worst case scenario, and then add another 10-15k to be safe, but i would also have your attorney draw up a letter stating that the seller is responsible for any cost, repair, damage, fines, incurred due to the oil leak in the ground.
Also make sure the neighbor property has been checked for any leakage from the sellers oil tank.
On many policies, the insurance covers the neighbor on the left and right but not the actual property...Once you get the paperwork from the DEP you should be OK..
If you like the house, and don't want to lose your rate lock ..it can be a hard decision...
Find out what will be your new rate, if it will the same and you will feel more comfortable , then wait..