I’d think 20-30 hours warning would be sufficient to implement emergency “hardening” even if it meant shutting down the grid and temporarily cutting connections during the height of the storm.
Emergency generators could be secured for hospitals and other emergency services in that amount of time, and shelters could be set up for the populace. Happens all the time with ice storms and hurricanes.
I’d be willing to endure a day or two without power to avoid such a dire outcome as having large portions of the electrical grid destroyed. I do it once or twice a year due to normal weather events here in the southeast anyway. What’s once more?
I believe that it doesn’t matter if it is plugged in or not.
In terms of infrastructure you would be fine but some businesses have systems that need 12 hours or more to properly shut down.
It does not matter if the device is “plugged in” to the grid or not. If it has conductive components, like wires, it can be fried. EMP is fairly poorly understand by most people, just like other forms of radiation.