Kurt Deibner is the obvious candidate to lead the German project in the absence of Heisenburg IMO.
Trouble is Heisenberg was by far the most respected nuclear scientist Germany had. When he said a nuclear weapon couldn't be built fast enough to affect the war, that had enormous weight and the budget/priority of the project dropped to very low.
BTW his calculations weren't a mess. Heisenberg was a bona fide genius and one of the preeminent nuclear physicists of the 20th century. As I understand it, he came up with a very elegant solution to the problem of calculating the critical mass, based on the assumption that the chain reaction must complete before the first neutron leaves the critical mass. The solution was correct, the trouble was, the assumption was wrong.
You also still have all the other problems that putative Nazi bomb projects have:
- The lack of Heavy Water to act as a moderator in a reactor.
- The failure to recognise that very pure graphite can replace Heavy Water.
- The general lack/competition for resources.
- The Allied espionage/bombing/sabotage campaign against the German nuclear programme.
I'm not saying it's impossible, but you need some major PoDs not just a theoretical breakthrough on the calculation of the critical mass. The British correctly determined the critical mass in 1940 and it still took the combined might of the British the Canadians and (not least) the Americans five years to get a bomb.