The fuel filter is just a strainer in the tank, not likely to have been changed, but if it was blocked wouldn't it be giving an error for running lean?
I believe the vac solenoid is to open the second intake on the air resonator at higher revs.
Another question, if the timing belt had been put on one tooth out, could that give the errors we have? Presumably the valve timing being out would affect both intake and exhaust flow?