Given this potential, why did the U.S. enjoy a tornado drought recently?
Climate change does not change the weather or patterns of weather (much). Last year there were anticyclonic conditions and subsiding air aloft, and the jet stream was pushed way north. There are almost no storms under those circumstances. The variability from year to year is large and the El Nino phenomenon plays a big role. This year El Nino or La Nina is not in play allowing the weather to be more normal and variable. The pattern is not locked in place as it was in the past two years (in very different ways).
Is the problem climate change or is the problem development in the path of extreme weather, whether in Tornado Alley or coasts exposed to hurricanes and the like?
No doubt the biggest issue is people putting themselves in harm's way, building in coastal areas and flood plains particularly.
What can we expect in the future given ever-increasing levels of greenhouse gases in the atmosphere that will presumably trap even more heat?
The heat has to go somewhere. We expect more extremes in the water cycle in particular. Stronger droughts, bigger heat waves and much greater wildfire risk, but also stronger storms and heavier rainfalls where the rain is occurring. Managing water will be a major challenge.