I don't see the problem here. If the vehicle has previously been issued with an import approval under the old 15 year rule regulations, then this import approval is still valid. If you have already checked with DOTARS as you have said then all you would need is a copy (or re-issue) of the import approval from DOTARS. I am not sure if they will re-issue an import approval to anyone but the original applicant, but a call to DOTARS can confirm that. However, be careful that the car was not approved to be imported for RACE USE ONLY. Again, DOTARS will be able to confirm.
Then all you would need is a RWC and compliance (to meet the 15 year rule regulations, not SEVS) in order to register the car in your state. You don't need the japanese de-registration papers, only a reciept of purchase from the guy you bought it from.