>The usable energy is proportional to the cross-section of the borehole

I mean we thought oil the same way until we developed fracking. While it's not likely a 1:1 relationship with fracking, I'm sure some clever geologists/drilling experts could massively increase the surface area available for heating given the right incentives.

As for facilities, the volcano eruption rate is rather low on human timescales, especially for effusive eruptions that don't tend to blow the entire area to bits. Getting 50-100 years between eruptions that could possibly total the facility is likely worth it as you don't have a direct fuel cost.