The developed world will use the first wave of immigration to build the walls that keep out the rest

Japan is way overpopulated. In the case that agriculture stops producing as much as it does now and food imports also dry up, Japan will be a very bad place to be. The country is mostly forest and mountain, arable land is rather scarce.

There's two main reasons way this doesn't scare me that much:

1) Japan's population is falling and I expect it to have fallen further, maybe to half or two thirds of current, until a global economical collapse.

2) There are ways to heavily increase the land use intensity with modern technology, which I'd expect to happen when the situation gets dicey. The Netherlands are a good example of how this can be done, essentially green houses everywhere. The actual bottleneck is how much energy you have, not the amount of land - Japan has lots of space in the sea for wind and solar energy, plus lots of unused geothermal.

I would be interested on more reading on these topics (collapse, human migration, global inequality, defense, survival etc.)

Two very expensive countries