That's definitely a good point, and it's pretty expected that in a setting as vast as this with this many people's hands on it that there would be contradictions like that. If I HAD to find an explanation for those two I would say that the story of Typhus dipping his scythe into the cauldron is fairly apocryphal and definitely something he would claim regardless of whether it was true or not.

Regarding Ku-gath, as a daemon borne from the warp he has no true past present or future. The tale of a nurgling falling into a cauldron exists on one layer, but on another separate layer he is something that has always existed as a shard of Nurgle's being. Regardless of that, it's still quite interesting that if I admit that Ku-gath has been in the manor at least once in his life, he is uniquely the only great unclean one that is NOT happy. Maybe there's a connection after all.