Why isn't "thirty days are in the month" also a correct translation?
It’s accepted now (2015-08-23).
Why isn't it pronounced tree-ocha?
Different dialects have slightly different pronunciations for some words. Here is tríocha in the three main dialects.
The O is present only to ensure that the CH is pronounced broad.
what about "the month has 30 days"?
Why is it 30 days in the month that's required, when it's "sa" rather than "san"?
'sa' is used before consonants, 'san' before vowels. They both mean 'in the'.
'In' without an article is 'i' (in Cork = i gCorcaigh)
"Narrows it down."
So its either September, April, June, or November.
No native speaker of English would say that. It's not idiomatic. A month has 30 days is correct.