@Adam Knight Why Japan would be screwed even with the agreements with the oil and farm country.
Fuel:
Japan's fuel reserves without importing would only last a few days, or a few weeks at most. And it takes many, many months and years, even to build oil infrastructure. This means in the mean time, people would probably be starving after a few weeks without any more fuel or effective means to transport it. Japan would have to do an abrupt manufacturing change (which takes months or years), then build the stuff to get the oil (even more months, with a rushed production), and then months for the infrastructure to transport the oil back to Japan. So in the mean time, Japan has huge power outages for a few years, massive starvation and deaths in probably the year (or more) it took to get the oil into a usable state.
Food:
Japan only has a food self sufficiency of about 50%, which means, even if they increased the food production, maybe would reach 60-70% (they have 40% the population of the US on the land size of California, even if they switch to as much farming as possible, the land size just isn't enough), so many people would have already starved before the food is transported back to Japan. And as for food imports, that isn't even worth considering. Assuming the agriculture country does all the transportation, that would take many, many, months to get to Japan. And even if they use all their self-produce able gas to transport food (unlikely as no trains, and trucks driving on rough terrain = dead trucks), they don't nearly have enough fuel to get sufficient amounts of food back to Japan before large amounts of the population died from transportation.
Social Structure:
Many people would be out of work (you can't hire people at factories/farms that don't exist yet). Huge increase in crime (for obvious reasons). Social Unrest. Mass Panic. Very, very strict food control, fuel usage, power usage. Lack of working hospitals or other medical needs too.
The author severely underestimates the dependency Japan has on external forces and established infrastructure as well as over estimates Japan's self-reliance/self-sustaining. Infrastructure is the single most important thing that most people (and the author) take it for granted, making any of this stuff unrealistic in the short time it took Japan to do it.
What I'm say is that if Japan really did suddenly get isekai'd as a whole, they better hope they teleport directly in between a sea of oil and an expansive plain of wheat/rice or else they are royally screwed.