What is the geothermal potential of a volcano?
Volcanoes erupt due to increase in pressure, within the magma chamber (often 3-10 km deep), due to exolusion of volatiles. The magma chamber is usually heated from below. Geothermal plants in volcanic regions exploit heat from hot water in the "shallow subsurface", few hundreds of meters deep (and rarely exceeding 1-2 km). So as far as your second last question is concerned the impact on the volcano will be little as deep hydrothermal circulation can only transport so much heat.

There are many such plants around the world, e.g., Iceland, Philippines, Indonesia, New Zealand etc. However drilling in hot rocks has its own set of problems. It is also not easy to build large plants near volcanic areas due to lack of accessibility, infrastructure, market etc.