Based on your description of the situation, you appear to have a difficult client with no apparent intention of compliance with SPCC requirements. You may be able to get around the date issue simply by referring to the original plan dates and reference to the client's "verbal confirmation" (if provided) to implement corrective measures. HOWEVER, from your description there appears to be some larger issues.
- The double-walled tank requires an "environmentally equivalent" method of integrity testing. Per EPA Guidance for SPCC Inspection (2013) this can be done by inspections - which are documented and should be part of the original SPCC plan supplements. Have they done inspections and internally identified issues and/or concerns that need to be addressed (e.g. staining; spillage; sheen on water in collection system; etc.)
- Have they had any reportable spills?
- Did the sheen you noticed on water in the storm water basin reach any surface water?
- Do they have a written procedure for refueling the diesel tank? Does it include ""active measures"" for spill / pollution prevention? If so, is it used? If used, why is there staining?
A few weeks ago I completed an SPCC Plan re-certification for a client that had a lot of environmental awareness. Although not originally anticipated, I had to rewrite most of the plan and, as required, provide a table showing where and what changes were made. Many of the changes I made (and clearly identified) were due to a certain degree of neglect by the original engineering SPCC authors (looked like a template was followed). I had no trouble re-certifying THE CHANGES I MADE to the original and ensuring the document was in compliance with appropriate regulations (and EPA Guidance).
Bottom Line: You should be able to re-certify an SPCC Plan Revision, assuming the revised document is consistent with regs and your observations. Remember you are certifying the Plan (and your observations). An authorized representative of the Client must sign and certify. The Client is ultimately responsible for following the Plan.