https://app.electricitymaps.com/zone/IS?wind=false&solar=fal...
Not the case in Iceland. Only 15% of the demand is non industrial, and people still use electricity regardless of what they're working on. Grindavik itself isn't large enough to be more than a rounding error.
>even without a dip in demand from that there’s usually some oversupply with electricity generators to avoid ramp up time problems during peak demand, so possibly they decided to shut down unused capacity to reduce the risk of it being damaged
No, geothermal plants generally aren't ramped, and they produce at maximum output unless there is maintenance. They may have reduced overall output to enable individual turbines to be turned off for brief periods while installing new remote control devices. That is speculation however as I'm not at work so I can't say for sure.
As much as it would suck to lose a power plant (we are already on the edge of energy shortages), it is far worse to lose hot water supply to Keflavik. They'd have to quickly build up electric heating capacity, which will make energy shortages.
And even if they interrupt service it's not like they aren't expecting service interruptions anyway (given the whole imminent volcano eruption)
There also GNSS and laser interferometers that watch for displacement of land.
Early warning and prep is key. And this isn't an 'emergency', yet. It's an 'Alert', to prepare people for the worst.