If you need to look up words or definitions of hawaiian words, www.wehewehe.org is a better resource than wikipedia...
ʻo ia - he, she, or it kāua - you and me (2 people) kākou - everyone (including speaker) lāua - they (2 people) lākou - they (3+ people) māua - us/we (2 people, not the person to whom you are speaking) mākou - us/we (3+ people, not including the person to whom you are speaking)
And kakau is a completely different word.
They is not always plural especially when referring to someone of transgender who's pronoun could be they. In this case they = it. So it fits the definition in Hawaiian for a pronoun but you're right it can also be plural and in that case you'd use the lākou you mentioned. It's more "ia" fits the gender neutral "they" so when looking at meaning instead of only having he/she, the use of they should be appropriate when translating from Hawaiian to English especially when no context of gender is present. "Hau'oli 'o ia" = "he/she/they (singular) are happy"
the glottal stop or ‘okina separates vowels, yes, but it seems like you are confusing which ones.
"Hau'oli 'o ia." means that there should be a break between the u and o within Hau'oli as well as between the i of Hau'oli and the 'o. The word 'o ia has all three vowels glided together. Otherwise, if there were a glottal break between the o and i in 'o ia, it would be written as o 'ia instead. Is that clear?