It is physically impossible that Mitsunari became Hideyori's father because Hideyori was born in August, 1593 and Mitsunari had been on an expedition to the Korean Peninsula since the previous year.
秀頼は1593年8月生まれであり、前年から朝鮮半島に赴いていた（文禄の役）三成が淀殿に子どもを産ませることは物理的に不可能である。