i know little about the mushroom war but I know it war it is in our time period and that’s how the mutants on the episode Susan strong and the opening theme and the nuclear bombs probably make PB a hybrid of human/bubblegum and Finn’s family probably hid and did not mutant and this happened when Marceline was a kid in memory of a memory and in the back round there was fire on houses probably from the war. Long after the mushroom war the rainicorn dog wars and Jake said that rainicorn dog wars were 1000 years ago. Pen ward confirmed that Marceline survived because of her super natural powers. I think the mushroom war blew a chuck off earth. if you know more please comment or post a pic of an item found in our period