My earlier point was that humans have already changed the earth. A plan or an attempt to mitigate the forces of nature is not an all or nothing proposition. Certainly any large geoengineering project involves financial and moral considerations. First, however, some back-of-napkin math is required.
Managing volcanoes using firefighting techniques may seem far-fetched, but its has already been done: ”... in 1973, an eruption on the nearby island of Heimaey [Iceland] threatened to bury a town’s economically important harbor; residents sprayed the advancing lava with seawater to cool it off, and the harbor remained intact.” (From ScienceNews). Roughly, eruptions measuring millions of tons of hot ash or lava might benefit from intervention with water, although too little water could make things worse, like wetting sauna rocks. Supervolcanoes, like Pinatubo, eject multiple cubic kilometers of ash and lava, and could not be managed.
As far as trying to control volcanoes or tap geothermal energy having any effect on the molten interior of the earth, there is nothing to worry about. There is a trillion cubic kilometers of earth. Even a thousand-years-long concerted effort to cool the mantle down
could only result in a tiny fraction of one degree temperature change, within a standard deviation of the interim mean.
Geoengineering and “new energy” ideas should not be too quickly embraced or rejected. A plausible idea should be alternately challenged and modified until it is proved unworkable, set aside for later study, or implemented.
For example, using lye and lime to scrub significant amounts of carbon dioxide from the atmosphere is economically impractical at this time, but the method is nonetheless quite feasible. There is a couple of trillion tonnes of CO2 in the atmosphere, a large but manageable amount.
Similarly, rising sea levels could be managed on a macro scale by pumping sea water (possibly desalted) onto the islands of the Canadian Arctic Archipelago. Each 500 cubic kilometers put on land lowers the sea by a whole millimeter.