Replace whole roof or half roof??
More than likely, due to the insulation being so tightly packed in each of the rafter bays, the wet insulation and plywood you discovered, (through the proper diligence of your roofer knowing to look for it), and also due to not having continuous intake and exhaust air flow to ventilate the spaces, the entire moisture problem is insulation and ventilation related.
The reroof 10 years ago did not create this situation.
The plywood on any side that the insulation is completely packed too tight in must be removed, even if only one side is showing the effects now, the other one will down the road.
The wet and moldy insulation should be removed and replaced with new. On top of the insulation, you need to have insulation baffle vent chutes installed.
At the bottom eave edge of the roof, either the entire soffit needs continuous length strip intake ventilation, or install Smart Vent, by DCI Products, to provide fresh air intake under the first courses of shingles.
In the top of the roof, it will need a full length of ridge vent, and I recommend the Shingle Vent II made by the Air Vent Corp.
Also, you should "STRONGLY" consider putting on at least a 30 year architectural style shingle instead of a cheap thin 25 year shingle.
One final note, gable vents will not work at all in your roof design, because of the enclosed rafter bays. It is not an open attic area, so that information previously provided was incorrect.