As I've said and always say, "In the end, it's your map and you can do whatever you want with it ".
That means if you need ash somewhere for your story, put it there. If you need grasslands and plains to support your people, livestock, agriculture, put them where you need them. Those kinds of things are not particularly dependent on hard geology such as plate tectonics and volcanoes, anyway. I
f you want a climate zone somewhere, put it there. There are plenty of maps with frozen zones in the mid-latitudes, caused by some magic. If you want them like Earth, here is climate zone summary of Earth:
http://eusoils.jrc.ec.europa.eu/proj.../CLIM_ZONE.gif
Just go and do likewise.
If you want to know what real volcanism does, read about the eruption of Krakatoa in 1883 and the "year without a summer". http://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa
or what Mt St Helens did across the US.
There are two kinds of volcanoes, shield volcanoes and stratovolcanoes, and they produce very, very different kinds of effects. Which kind is yours? And see how this brings us back to realism again?
So yes, you are probably overthinking it. With some non-realism, you might as well just use as much as you need for your story. Your readers already have to suspend belief, so more is not that much of a stretch. Make your volcano do whatever you need it to do for the story.