> And if you pronounced [e] and [ɛ] to native speakers of either Spanish or Japanese, they most likely wouldn't be able to differentiate the two sounds consistently without having had training.

I am a Spanish guy living in Japan, so I can confirm it. I didn't have any idea that the two sounds are supposed to be different because to me they sound exactly the same.

Fun fact: Japanese people are surprised that we have the same five vowels (although the U is a bit different), and that we can get the correct pronunciation very easily.