I think all your math looks good except for one thing: the natural gas genset you linked is probably more efficient than this plant because the plant uses a boiler. These purpose built engines can be much more efficient than a boiler. Side note: there's still a natural gas boiler here in town with a water outlet into the lake. I've swam through it, and it's incredibly warm. Like warm bathtub warm, even when the rest of the lake is mild. Thankfully the plant doesn't run much anymore.

A couple thoughts overall:

I don't think the local community can do anything to stop buying power from them. From what I gather from the article, the generator is connected to the grid but from a metering perspective outputs nothing. Generators have always had auxiliary load, especially coal and nuclear plants with big pumps and pulverizers. Those are all netted with plant output. It seems they have their miners all hooked up on the plant side of the metering. They are selling power to nobody but themselves.

The way to kill this plant is to eliminate once-through cooling. Make them build a cooling tower and the plant will die overnight.

About wind farms, and solar will have the same problem: the capital cost of the miners is a major expense, so they want zero downtime. While they could net against a wind or solar plant, they'd have to stop mining when the wind dies down or the sun sets. They could buy power from the grid, but now they expose themselves to utility pricing which could open them up to higher rates for crypto mining.