but then what about 'a hull' being her apple, not his? (pardon the lack of accents)
The prefixed H in a húll isn’t an example of lenition; the leniting H always follows a consonant.
It's because úll starts with a vowel, and portán with a consonant.
You should take a look to the Tips and notes of Possessives, it's clearly explained. I personnally read it when i'm confused.
You can open the web browser on your mobile and access the tips and notes there.
I have also been told, that something (a pronoun or a preposition) either lenites or puts an 'h' in front of the vowel of the next noun - but no preposition or pronoun causes both.