We've been told many times that Gàidhlig doesn't like vowels next to each other across verb boundaries, but here that's what happens.
Is there a reason for not using nach, or is this one of those irregularities that all languages have?
(My guess: a is broad but i is slender, so that's why it's allowed?)
It cannot be nach because it’s not the correct word grammatically.
- na is the negative imperative particle (some dictionaries say it’s an adverb… but it doesn’t work like other adverbs), it means don’t (do)…!; it’s for giving negative orders, for telling somebody not to do something;
- nach…? is the negative interrogative particle, it means roughly doesn’t…?, isn’t…? and is used to ask negative questions; to ask whether somebody didn’t do something.
As for the vowels disappearing – not entirely sure, but I can tell you that /i/ in ith cannot disappear as it is stressed (and stressed vowels don’t disappear). If /a/ in na also gets the stress – then that’d be what’s preventing it from disappearing as well.
(And I guess that’s what happens since the pronunciation for the imperative na given in AFB is /na/ with full /a/ vowel, while eg. for na in the meaning of or is /nə/ with reduced never-stressed /ə/ instead.)
(Also, apparently, in the past tense, an do can be shortened to na, eg. instead of an do rinn sinn e? did we do it? one may say Na rinn sinn e? for the same meaning – but note that here it’s not negative, so unrelated to the imperative na and the particle nach.)