It sounds to me like you have the best plan for that. 18 infantry makes for a good defensive wall and you are right, by the time the Japs get that far, they are using most or all of their planes far away. It sucks to give up those far eastern IPCs, but it’s a good way to keep the Japs from driving even further into Russia and actually being some help to the Germans and Italians. Plus, with Germany punching it’s way toward Moscow with Italian help, you don’t want to be expending valuable units messing around in the east when you have your major cities to defend.
Don’t do the same mistake that our Russian player often does. He brings back those 18 infantry out of range of the Japanese air force and any major reinforcing from Japan. Then a rather small Japanese force shows up, perhaps 3-5 units, usually just a few infantry and artillery. Then he attacks that small Japanese force with his 18 infantry and no other support. Yeah, he usually annihilates the small Japanese force, but he actually takes more losses to his own force. After about 2 or 3 times doing this, that big 18 infantry stack is reduced to 5 or 6, sometimes even less. Then Japan rolls up with a few tanks and BAM! No more defense perimiter. Now inner Russia is open for those Jap tanks to blitz all over. Not only that, but if the Jap tank force actually reaches Moscow, they may suicide themselves just to cut down some of Moscow’s defense and make it a little easier for the big German attack.