It is hard to claim with 100% accuracy the activities of Subhas Chandra Bose between 1946 to 48. However, some facts and logical analysis can help us in constructing a theory. First, Deben Sen accompanied by Joglekar saw Subhas Chandra Bose in Marseilles airport in 1946. Around the same time De Valera, the president of Ireland, and a great friend of Netaji, was ill. British communist leader Gallacher claimed that Bose secretly visited UK and Ireland. Bhagwanji claimed that he met Will Durrant, the famous historian, and Ariel Durrant secretly, with a specific request. Apparently, Will Durrant was surprised that he took such a risk just to serve his motherland's interest. Ariel Durrant said that for the sake of his motherland he would be risking his life a thousand times. The meeting was a secret one. Therefore it can be surmised that Netaji went to Ireland to meet De Valera and also met Will Durrant. It can be surmised that some section of the Russian Communist Party passed on the information to the British Communist Party. British now knew that Netaji was in Russia. Their suspicions were confirmed. They possibly put up pressure on Stalin to arrest and kill Subhas Bose. The allied forces had the atom bomb, and Russia had no atom bomb till then. The first atom bomb was tested by Russia in 1949. Under the threat of a nuclear war, Stalin possibly held Subhas Bose captive for sometime in a Gulag. The Russian government also fabricated the story of the ill-treatment of Bose in prison, just to placate the British and the Americans. Therefore it was chronicled by some Russian scholars that Subhas was in a prison and he was ill treated, while in reality Bose possibly enjoyed a better status, but was kept under watch. There are claims that Vijayalakshmi Pandit saw Bose in a pitiable condition. This is not credible. Whatever be the attitude of Ms. Pandit towards Bose, she would have done something for him had he really been mistreated by the Russians. The same goes for Dr. Radhakrishnan, a spiritual person. He would not have remained quiet had he seen Bose in a prison. Therefore they must have met Bose but in an agreeable condition. What Ardhendu Sarkar's colleague told him, confirms that assumption, that Bose was under surveillance by Stalin but was given due respect. Bose had planned to return to India in 1948-49. After Russia tested Atom Bomb in 1949, Stalin had no further obligation to detain Bose for fear of the US or UK. He let him go and Bose was in China, at the head of the Asiatic Liberation Army, helping Mao regain power through a Communist revolution. This was reported by Sarat Bose who got the news from his own sources as well as from the European reporters, in The Nation. Maoist forces stormed Nanjing in April 1949 and Kaishek fled. But The Indian Government under Nehru, with active assistance from British MI6, had closed all entry points for Bose, so Bose, despite his plan, could not cross over to India in 1949. Subhas Chandra Bose was "dead" to his beloved Nation, and he realized the extent of Nehru's treachery, betrayal, by the early 1950s. A section of the Russian communist party possibly hoodwinked Dr. Satyanarayan Sinha in believing that Netaji was being tortured and killed in Gulag.
What were Netaji's whereabouts between August 1945 and February 1946? He gave three broadcasts from radio Manchuria between December 1945 and February 1946, that were picked up by P.C Kar. But before that? As per Dr. JBP More's testimony and also from other anecdotes, Netaji was active in Indochina, working closely with Ho Chi Minh for the freedom of Annam or Vietnam from the French. The French forces tried to capture him and severely tortured Purachandi, Dr. More's stepfather, but possibly couldn't capture Netaji. They combed the jungles of Indochina and Siam, sending paratroopers and bombing that area for Netaji, yet, failed to nab him - These are the excerpts from the French intelligence report as deciphered by Dr. More. So between August 1945, and December 1945, Netaji took great risks and came to Indochina to help the Vietnamese fighting for independence from the French. After December, he possibly was stationed in Manchuria and later he moved over to the Russian side.
Why did the French not capture him in Marseilles? Netaji was protected by the Russians, having Russian bodyguards commissioned by Stalin and the French could have risked a confrontation with Stalin at that stage.
All this is a theory with loosely fitted facts, but they are based on logical premises. The truth may be different, but there may also be considerable overlap with this narrative.