Even if it feels wrong to base it in Washington D.C., but you probably have to do that for logistical reason.
Yup. To be safe, I checked the public transit times to Charlottesville from Philadelphia and Baltimore before submitting DC as the hotspot center, but neither Philadelphia or Baltimore worked out to allow transit one way in four hours or less. As for whether DC should ever get a World Heritage Site, I've probably visited the Mall too many times to have an unbiased opinion of its OUV; I don't particularly see it, but I appreciate outside perspectives.
For potential future US hotspots, should New York City gain one more World Heritage Site, or should Bethlehem, Pennsylvania's Moravian district be added as an extension of Denmark's Christiansfeld, both Philadelphia and New York City would be eligible as hotspots on the US East Coast.