Re: If Hitler converted to Islam before his death, would his sins have been forgiven?
We do not know what Hitler would have done, and we don’t know whether Hitler’s actions would have fitted the requirements for a true repentance. Yes, we’re told that a true repentence renders past sins forgiven, but how is it decided whether a person truly repented. So, taking a specific person, and asking whether that person has been forgiven is pointless, because we’d never know how sincere the repentence was, and under what circumstances it was done (like you mentioned, a repentence done knowing death is near might not be accepted).
We can speak in generalities, and say one should always do dua, etc etc. But us making judgements on another person is ridiculous. This hitler incident never happened, and even if it did, we still can’t be sure of what God would think of Hitler.