Trying to keep my baby alive! HELP please HELP.


I have a 2005 Honda Pilot EX, fully loaded, with 148000 miles. I bought it used with 135000 miles 3 years ago to date, and I want to keep it to at least 300000 miles (I love this car).
I assume it was owned by a home mechanic (I got it from a dealer), judging by the fact that he put new brake pads on badly grooved rotors (which I replaced with cross-drilled ventilated disks and ceramic pads and flushed the brake fluid). And judging by the lack of an indicator sticker (date Mileage), he most likely changed the timing belt and pump.
Which brings me to my problem, the car always starts without hesitation and passed emissions (in hot or cold weather, I live in Chicago), it seems to run ok, idles at about 800 rpm, and accelerates semi smoothly, and quick. Yet, I have a check engine light on, and when scanned I get misfire codes on all 6 cylinders. For which I changed all spark plugs, all ignition coils, my mechanic said my wires are ok (flow and resistance), There is no blow-by, it burns no oil, and the temp never variates.
Now my mech tells me that the only way to figure out what causes this is to perform exploratory surgery on the engine (to the tune of $450).
Hence my questions:
What could possibly cause a misfire on all six cylinders?
Could the HmMec, have possibly advanced the shaft by a click-forward or back by mistake (thus advancing or retarding timing) before putting the new belt on.
And if this is a computer problem, how could I check for that?
Due to the fact that I'm a furlough school bus driver, $450 is about 1/2 of my monthly income, which makes it hard to part with.
Please help.