I wrote "a núll" and it told me I had a typo. How important is it, orthographically, to write it as "a n-úll" as opposed to "a núll" or "a n'úll"?
Is it roughly akin to "color" vs "colour" where it's a regional thing? Is it roughly akin to "Brian" vs "Bryan" where there really is no standard? Or is it a proper error and Duo is just being lenient?
The purpose of the hyphen is to tell you that the n is a prefix, not a part of the word - a n-athair and a nathair do not mean the same thing. For this reason, you do not need a hyphen when a word has a capital letter, because the prefix remains in lowercase - Tir na nÓg. The same is true for the t- prefix before vowels, but not a h prefix, which doesn't hyphenate.
One of them has a h in front of it and one has a n-prefix.
húll and n-úll can't exist on their own - the initial mutations only occur as a result of the previous word. In the case of the possessive adjective a, it can mean "his", "her" or "their", and the following would is modified to clarify which a is being used.
a úll - "his apple"
a húll - "her apple"
a n-úll - "their apple"
ár n-úll - "our apple"
bhur n-úll - "your (plural) apple"
úll, húll and n-úll all just mean apple, just as "apple" and "Apple" mean "apple", but "Apple" is only used in particular grammatical circumstances.
Apparently, not everyone recognizes "y'all" as a plural only form - it is sometimes used as a polite singular form, which is never true of sibh or eclipsis after a. And some people would say that you need "all y'alls apple".
Many varieties of spoken English in can differentiate between "you/r" singular and "you/r" plural, but that distinction isn't usually formalized in written English.