“but for an actual first-language Yiddish speaker it would be quite odd to spell it in a way that it too similar to “nohr/nur” (“only”) ”
Ive been trying to think of an example here is one: what does פאר mean?
Without context (or sticking in a nekuda) you dont know. Here are two contexts to help:
איך פאר אהיים (travel)
איך מיז זיין אהיים פאר די זמן (before)
(of course the word זמן is technically Hebrew but like the word שבת it doesnt contain any yiddish vowels so it is left alone)