For the record, I’ve yet to see Calcutta saved unless Japan chooses not to pursue it.
Even “optimal” play by the Allies to save Calcutta is a sub-optimal strategy because Europe is almost guaranteed to be lost. Saving Calcutta is not worth giving up the game in Europe.
I think this is the point Elk is making Simon.
You may be able to save Calcutta, or make it severely expensive to do so, but in the 4 years I’ve been playing global, the only time I’ve failed to take Calcutta as Japan is because I did something else like attack Russia instead or sack Sydney early. I have saved Calcutta by misplays or poor calculations by my opponents, but that isn’t classified as optimal play.
Sometimes the losses for Japan are prohibitive which are the nature of dice games, but I’ve yet to see Calcutta saved by playing it against my own strategies or against human opponents. I’ve stacked Yunnan like you’ve suggested for J3 showdowns and Yunnan still falls with very little left to defend Calcutta, I’ve turtled, and I’ve even done the full Allied investment into saving Calcutta multiple times (MIC in Iraq / Persia) but it forsakes Moscow and turns the game into a meta game of playing for Egypt in sub-optimal conditions for the Allies due to proximity of production as Europe quickly becomes an impenetrable fortress for the Axis.
In my opinion there aren’t many viable strategies for India except to get a MIC in the Middle East for London to help fly planes to Moscow and then hunker down to keep Japan bogged in a sub-optimal location as the Allies assert control over the Pacific.
You can argue otherwise, but my experience leads me to believe that the hundred or so games I’ve played gives merit to my assertions. You may have alternate strategies for a more effective India, or a more annoying one, but in my book Calcutta falls to Japan if I say so or I get diced.