For a very long time the Pacific Islands have been recognised as being a high-risk area, so yes there is a severe risk. I am afraid for many of them, the only pragmatic option may well be to evacuate. In terms of the border-lying countries, it really depends on where you are and the resources you have.
Some countries are well off enough to support sea defences and defences of that kind.
So the Pacific Islands really are more at risk. Some may, but definitely not all. The amount of warming that we have had over this past century has been about 0.75 degrees Celsius, I think. In the global meaning, yes. The global average temperature of 3 degrees - 3 to 5 degrees.