Firstly Lilias is a Scots, not a Gaelic name so none of the spellings are more 'Gaelic' than the other. As an old name it arose at a time when spellings were fluid and I have an ancestor who goes through pretty much all of the variants during her lifetime. I wouldn't therefore say any of those spellings are more legitimate than the others, although in my experience the commonest form is Lilias. I definitely haven't seen Lileas as much. As to pronunciation I have only ever heard LIH-lee-us, and my ancestor also appears as Lily which would back this up.