a lot of the doubts were because only the Soviets, with some German help, saw the bodies burnt.
"Various versions of Hitler's fate were presented by the Soviet Union according to its political desires. In the years immediately following 1945, the Soviets maintained Hitler was not dead, but had fled and was being shielded by the former western allies. This worked for a time to create doubt among western authorities. The chief of the U.S. trial counsel at Nuremberg, Thomas J. Dodd, said: "No one can say he is dead." When President Harry S. Truman asked Stalin at the Potsdam Conference in August 1945 whether or not Hitler was dead, Stalin replied bluntly, "No". But by 11 May 1945, the Soviets had already confirmed through Hitler's dentist, Hugo Blaschke, dental assistant Käthe Heusermann and dental technician Fritz Echtmann that the dental remains found were Hitler's and Braun's. In November 1945, Dick White, then head of counter-intelligence in the British sector of Berlin (and later head of MI5 and MI6 in succession), had their agent Hugh Trevor-Roper investigate the matter to counter the Soviet claims. His findings were written in a report and published in book form in 1947.
"In May 1946, SMERSH agents recovered from the crater where Hitler was buried two burned skull fragments with gunshot damage. These remains were apparently forgotten in the Russian State Archives until 1993, when they were re-found. In 2009 DNA and forensic tests were performed on the skull fragment, which Soviet officials had long believed to be Hitler's. According to the American researchers, the tests revealed that the skull was actually that of a woman and the examination of the sutures where the skull plates come together placed her age at less than 40 years old. The jaw fragments which had been recovered in May 1945 were not tested.
"In 1969, Soviet journalist Lev Bezymensky's book on the death of Hitler was published in the West. It included the SMERSH autopsy report, but because of the earlier disinformation attempts, western historians thought it untrustworthy."
The full story, which seems well sourced (though I haven't checked) is here
https://en.wikipedia.org/wiki/Death_of_Adolf_Hitler
Whether properly cremated or not, and whether you believe the Russians or not, it sounds as if he was dead. But nobody at all witnessed the actual death.