If Japan wants to commit the resources it can take Calcutta on turn 4 regardless of what the UK does. The only way they would fail is extremely poor rolling. In my video all I laid out is how to set up Middle Earth. The challenge is what to do from there with the influx of British units in the middle of the board.
It takes all of your creativity and instincts to properly utilize your position. After securing the Middle East my preference would be to secure Africa and the Med. The only thing that would delay that for me is if Japan is making a move towards Calcutta. As you stated there is only one transport load of units from South Africa before J4, but there is also 3 units built in Persia and the air from the Med that can be shifted to India. What that gives you is 5 ground troops (at least 4 fast movers), any leftover units from taking the Middle East (2-5 units), plus the air (2-5 planes). I’ve had to deal with the Calcutta Crush before while playing Middle Earth.If Japan is going to Calcutta J4 then they will not have many ground units left over after the assault, leaving you the opportunity to retake it. There’s no guarantee you’ll get it back but if you don’t have all of those units there you will likely never get it back. If you do get it back then Japan will have blown it’s wad in the attempt and forsaken the Chinese, the Money Islands, and the Americans who now have the opportunity to turn the table on Japan. The naval base that I put in Persia allows me to throw some units (sacrificing transports) as far away as Burma and Shan State if the right moment presents itself. Bottom line: The Calcutta Crush is not a reason to not do Middle Earth because no other strategy is going to prevent it either.
As far as Sealion goes, please do Sealion if you’re playing against me. I will make that a short game. You don’t purchase your Persia complex until UK2 and by then you will know if Germany is doing an early game Sealion. Forget about Middle Earth and gang up on Germany. After you sink their navy on US3 and UK3, feed them a steady diet of Russian tanks, mechs, planes, artillery, and infantry. Support that with Americans and newly liberated British on the western front. Do enough to keep the Japanese from capturing 6 Victory Cities in the Pacific and burn down the Third Reich.
That would be London Calling;
https://www.youtube.com/watch?v=l0ZYQ-C4i28&t=3091s
The one time that my opponents did Sealion and Calcutta Crush on me simultaneously, the assault on India failed due to bad rolling and then I sank the German navy. It would have been a close game if India would have fallen, but it turned out to be pretty one-sided. You can’t count on that kind of luck every game, but then again if the Axis are completely fixated on the UK and determined to prevent Middle Earth it should leave the other nations open to coming up with a way to pull out a win.