From "Heisenberg's war : the secret history of the German bomb", Heisenberg realized in mid 1941 that what he thought was "safe" work in nuclear reactors could be used to produced bombs. https://archive.org/details/heisenbergswarse0000powe/page/11... and by that summer thought it was widely enough known to be a concern.

He hoped to "try to channel research in the direction of a power-producing machine" (https://archive.org/details/heisenbergswarse0000powe/page/11...)

"Whenever Heisenberg spoke of the visit [to Bohr] he always placed one question at the center of his concerns: what should the German scientists do? The dilemma was quite stark: if it would be wrong to give a bomb to Hitler, was it also wrong to ‘‘work’’ on bomb-related research? The answer was not easy. They might preserve their moral integrity by refusing to do research on fission, but in that event the Heereswaffenamt might only find other, more willing physicists to do the job. By continuing to work on the project, Heisenberg, Weizsicker and Wirtz would maintain a degree of control over its direction—so long as the military authorities made no decision to embark on a full-scale program to build a bomb. Any decision of that kind, Heisenberg knew, would deliver control of the project into military hands.”' - https://archive.org/details/heisenbergswarse0000powe/page/11...

By contrast, other Germans in 1942 were more enthusiastic. From https://archive.org/details/heisenbergswarse0000powe/page/13...

> In a report prepared for the Heereswaffenamt, Diebner and his associates—Friedrich Berkei, Werner Czulius, Georg Hartwig and W. Herrmann—claimed that a bomb might be built with an ‘‘explosive effect a million times greater than the same weight of dynamite’’ using either uranium-235 or the ninety-fourth element, produced in reactors.” By what route the general knowledge of plutonium reached Diebner and the Heereswaffenamt, and how long it took, is not known,” but it was discussed clearly in the report by Diebner’s group completed in February 1942. The report, Energiegewinnung aus Uran (‘‘Energy Production from Uranium’’), said 10 to 100 kilograms of fissionable material would be required for a bomb—the lower figure was in fact about right—and urged that the program be transformed into a major industrial project. The report did not minimize the effort required, confessing frankly that separation of U-235 was still beyond reach, and that the details of plutonium production remained to be worked out.

I believe your "Heisenberg indeed didn't believe anymore in the possibility of building a fission bomb" is best interpreted in the larger context of "in the timeframe the military wanted".

When you say "atomic weapons program" and "leader of the (unsuccessful) German effort to develop an atomic bomb", what specifically do you mean? "Diebner, throughout the life of the nuclear weapon project, had more control over nuclear-fission research than did Walther Bothe, Klaus Clusius, Otto Hahn, Paul Harteck, or Werner Heisenberg." says https://en.wikipedia.org/wiki/German_nuclear_program_during_... and https://archive.org/details/heisenbergswarse0000powe/page/14... gives more about what he did in developing that report.