Viability of Sealion aside, I think Japan’s best option to support a Sealion is to blitz the Pacific hard in order to either delay America’s liberation of London or take advantage of America’s focus in the Atlantic.
Russia is unlikely (and would be unwise) to send units east to save a handful of IPCs. While Russia losses 1-2 IPCs a turn, Japan is has to dedicate huge amounts of it’s land and air forces to chasing the Amur stack across Siberia. And what does that look like when you factor in build time and location? 2 less infantry on the Russia/German boarder on R6? 6 less on R9? It’s just not enough gain for the investment.
Every turn delay to liberating London, on the other hand, keeps Atlantic UK’s entire income out of the war. It provides 8ipc per turn to Germany, which it can use much more efficiently than Japan. It keeps cash cows like Norway in German hands. A monster Japan is itself a huge threat of a victory city win, and forces the Allies to react accordingly instead of pounding Germany.