RE: code P0306
The list of things that could cause a P0306 ( cylinder 6 missfire ) are somewhat endless caferriter.
Ignition system components such as spark plugs, plug wires, or coils are by far the most common causes of a missfire. The list, however, can also include problems with fuel injectors( leaking fuel, poor spray pattern, or just plain mechanical or electrical failure), low compression due to a variety of different problems such as burnt valves or collapsed piston rings, vacuum leaks from just about anywhere, and the list goes on. I think the fact that your plugs and wires were replaced two months prior and that fixed thecheck engine light during that timemeans the first shop was on the right track. If not the CEL would have probably come on again long before.
The question is, why does the dealer want to replace them again and are they going to guarantee that this will correct the problem. If the original installed parts were cheap and of low quality then I could see a failure in that short of a time. (You do not happen to remember the original code # that was stored do you? ) But what if the problem is that #6 cylinder is burning oil and plug can only survive for 2 months before being completely fouled with oil again? Throwing spark plugs at it is not going to solve the problem in the long term if that is the case. The dealer should be able to explain to your satisfaction just why these new parts are again in need of replacement. If they cannot, they have no business trying to diagnose and repair anything.
Would drive the van as absolute little as possible with the CEL flashing. The only time the light will flash is if you have a severe missfire that could damage the catalytic converters. The converters are covered for an extended time of 8 years/80,000 miles but do not know if you are close to or over those numbers. They are very expensive at over $700 per side and that is not including any labor to install them.