I think if they hadn't put the whole "I ATE THEM!" bit in the American dub, there would be less confusion. In the Hodgson story, the mold all over the island grows on the unlucky couple regardless - being forced to eat the fungus due to the starvation was horrible icing on the evil cake. The fact that Murai says that he kept both Akiko and himself from eating the shrooms, no matter how hungry they got, is good enough for me. Add to that the way
he says "for what purpose?" at the moment of the reveal, and I took it as unambiguous. The horror was unavoidable - it didn't even matter whether they ate or not.
Since the movie plays out as a metaphor for the social and moral decay inherent in the modern world, this interpretation is seems best and is the more thematically consistent. Struggling against the dehumanizing influence of modern society is, in the end, a pointless struggle - the decay creeps in inexorably. Just being near it all is enough to change you, permanently, whether you want it or not.
Regarding infecting Japan, they do mention early on how the fungus is very "susceptible to chemicals." A few rounds of Lamisil and I'm sure he'd be right as rain