Hi, welcome to Just Answers.
Sounds like you may have a problem with your EGR valve and clogged EGR ports in the intake manifold...this is a common problem on Accord 3.0L..has the EGR system ever been serviced in the past? On higher milage engine's, I've had to remove the intake completely from the engine to clean out the EGR ports. Basically what is happening is the EGR valve is staying slightly open and allowing the exhaust gasses to enter the cylidners, which in turn causes a missfire at idle.
Honda does not supply a diagnostic flow chart for this code. P1399 is a random misfire code and Honda/Acura has reported for various models that this could be caused by clogged EGR ports. In addition there is the possibility that the valve cleareance specification may be out of range.
To have a misfire across all the cylinders it can only be fuel related, computer injector control, cam timing, vacuum leak, or the EGR system...not ignition related since each cylinder has it's own ignition coil. The EGR system by far is more the reason for all these misfires at 130k, where as engine and fuel system problems are less likely. If the EGR valve was replaced at 45k miles,,,that's almost 100k miles ago...so the chances for another bad valve are great. But if the intake manifold has not been cleared through ever, this poses even more of a concern. Also, the EGR system is set to operate after the car has reached normal operating temperature...so, if the ambient temps are colder it will take longer for the engine to get hot enough to accept EGR gases, so it is possible to feel an EGR misfire in gear, at speed...
Don't replace the plugs yet..it would be rare that all the plugs would cause this. Check for any vacuum leaks around the intake with carb cleaner, and around the base of the valve. Remove the EGR valve and see if the engine will run with the valve removed...if so, than the intake is plugged. With the valve out, put some carb cleaner down the port and see if it comes out of the pintle, even the smallest amount...if so the valve is bad...
Let's check these items first, before we did into the fuel system.
Did the engine run with the EGR valve off? Have you removed the fuel filter out and poured out the gas in a clear container? Your theory on "bad gas" may be true here.
Yes, if Diesel was put in the engine would not run. Not even with 25% Diesel. It might run with very little Diesel and ping with no power. It sounds like you may end up with water in the fuel, bad gas, or still possibly the EGR system malfunctioning. Let me knoif you have any questions after they take a look at it.
You did an awesome job and it sucks that places can still not take care of their underground tanks and allow customers to get bad fuel. Imagine all the other people who fueled up their the last few days and are having the same problems as you. At least you know that because the dealer and us came up with the same conclusion, that you didn't get ripped off on any repairs. As for the test drive, I don't know why most technicians don't put the adequate mileage on the vehicle's, Aviation technicians don't let the planes out with passengers until extensive testing after repairs, should we do the same?? Great job on everything, let me know if you ever run into any trouble I can add my advice to :)