This all sounds very feasible. A large volcano venting over 10ís or 100ís of thousands of years must have a considerable effect on the global weather and temperatures. Two or more such volcanoes venting at the same time must be a distinct possibility and could have raised global temperatures quite rapidly. This, in turn, would have had a significant effect on funa leading to food shortages throughout the entire food chain.
We know that mass extinctions have occurred many times and not all could be attributed to celestial collisions. Indeed, not all mass extinctions are on the scale of even the KT event. These gaseous volcanic emissions could well explain some of the lesser extinctions. Given enough such geological ventings, perhaps even some of the more devastating extinctions could be explained.