Actually, this is the point everyone missed. The kind of volcanoes you get at spreading mid-ocean ridges are NOT the kind you get at Yellowstone or Mammoth Lakes or even Mount St. Helens.
As a geologist, I can verify that this scenario is pretty much ASB. The exact reasons go to the silica and water content of magmas from different sources, and the resulting variation in viscosity. Thinner basaltic magmas like the Icelandic ones are 'thinner' and contain less water, they also become more liquid with release of pressure (as on erupting). Explosive volcanism is therefore limited. Silica-rich, water-rich magmas like those found near subduction zones (St. Helens) or continental hot spots (Yellowstone) are far more viscous, and become more solid on release of pressure, and also cannot hold as much water dissolved in the magma as they come to the surface. The rapid exsolution of water vapor as bubbles, combined with the 'stiffness' of the rising magma, makes these far, far more prone to explosive events.
The current activity in Iceland is about as bad as it would realistically get. And the current cloud with its high base over Europe would NOT stop all combat flying in WW2, much of which was done lower, and the rest would just be pushed through anyway. We can build more piston engines. There IS a war on, y'know.
Iceland won't be affected like Clark after Pinatubo, because the Icelandic volcanoes are not capable of such an eruption. And the effects in Europe from any conceivable Icelandic eruption will not be much worse than what we see right now.