ETOPS flights are regularly flown 3+ hours from a suitable place to land (single-engine flying time). Considering they are up to 370 minutes (about 6 hours) in some extreme cases, I can certainly see this being a problem. If you had a 3h warning at any given moment, there's a strong possibility that some jetliner somewhere in the world is over an ocean and could not land in time.

The entire answer would be hard ... but depends on the longevity/intensity of the storm. The 1921 event is said to have lasted for over 24 hours (May 13-15). I'd guess that some effects would be 'spread' to an extent by the magnetosphere. [0]

According to WP the "plasma cloud ... compresses the Earth's magnetic field and thus increases this field at the Earth's surface."[1] I'd guess again that this is probably less of a problem away from the area directly struck by the plasma ... for radar, say ... but radio/sat nav effects might be worldwide. Some SWPC details:'Space Weather Scales': [2]

It'd be very interesting to see this question addressed in some detail.

[0] https://www.youtube.com/watch?v=mEqE-g128kk

[1] https://en.wikipedia.org/wiki/Geomagnetic_storm

[2] http://www.swpc.noaa.gov/sites/default/files/images/NOAAscal...