As a numeral (and prefix), is aon pronounced /e:n/ or /i:n/ outside of Munster? I’ve found both online, and I even found something that says it can be both.

More or less the same everywhere as far as I know. There might be variations I'm not aware of, of course.

Here's what teanglann's pronunciation guide gives for each:
https://www.teanglann.ie/en/fuaim/aon

It depends, I think it's /i:n/ in Connachta.
In Donegal, I heard 3 pronunciations: /e:n/, /i:n/ and /ɯ:n/.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

I’d say it’s
a haon /ə hi:n/ (C, U), /ə he:n/ (M)
aon áit /e:n a:t´/ (C, U, M)

compare
https://www.teanglann.ie/en/fuaim/aon_déag for (a h)aon déag
https://www.teanglann.ie/en/fuaim/aon_áit for aon áit

To me, "aon áit" isn't something you usually say in Ulster, to say "any", people rather use "...ar bith" there.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

I guess there is variation? This is what I’m talking about.

Is there?

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Please wait for corrections/ more input from other forum members before acting on advice

That's not what other people have been saying. Oh boy.