\e\ is a sound that is basically impossible to represent unambiguously to English speakers, without falling back to how it's used in words ('e' as in 'ten', for example).

`eh` and `ay` can both easily be read as a dipthong that would be represented as `ei` in any reasonably spelled language (like Finnish).

To keep things clear: the notation you'd typically want to use with IPA when taking about sounds (phones) is [e], as in this case, and use /e/ (with forward slashes) when taking about phonemes. Not to nitpick on you specifically, I see a lot of ad-hoc notations in these comments.

"ei" is more "ey" tho.

I think t-eh-n is pretty illustrative of why eh works better than "ay" either way - ay is much much closer to ey and so ei.

Depends on the language.

In some languages it's /ɛ/ (English "eh"), in others it's /e/ (Scottish English "ay"), and in yet others it's /e̞/ (halfway between the two). It's rarely /ɛɪ/ ("ay" in most dialects of English), though it appears in many other languages written as something other than "e".

Dutch is an interesting case: /ɛ/, /e/, and /ɛɪ/ are all different phonemes (short "e", long "e"/"ee", and "ei"/"ij" respectively).

Many only have one. Greek just has /ɛ/. Japanese just has /e̞/.