The simple answer to the question posed in the thread title is:
The authoritative source of Catholic teaching is Sacred Scripture and Sacred Tradition, which is preserved, passed down, and articulated by the Magisterium of the Church.
That simple answer requires some unpacking, of course. It’s easy to be clear on what Sacred Scripture is, because it is a tangible and finite collection of books. Sacred Tradition is harder for us to wrap our minds around and exactly who the Magisterium is and what documents are considered binding can trip people up.
The documents of the magisterium are primarily writings from popes and ecumenical councils.
Something like the Catechism, though it is a “sure norm for teaching the faith”, does not technically possess an authority of it’s own. It is authoritative in so far as it references Scripture and authoritative magisterial teaching. But it is not an exercise of the magisterium on it’s own.
Which is not to say, of course, that it can’t be trusted. It can. But it helps to be clear on what the purpose of a catechism is. It is the convenient packaging of magisterial teaching, but it’s not the source of magisterial teaching. New dogmas would not be proposed in a catechism. Those come about through exercises of the Magisterium: ecumenical councils or (particular) papal documents.
In general, it’s not particularly necessary or worthwhile for the average Catholic in the pew to spend a whole lot of time parsing out every line of every extant Catholic writing in order to come up with some Catholic “theory of everything” which will provide exacting parameters for every required Catholic belief as it applies to every imaginable lived scenario outside of which everything else can be summarily dismissed as so much chaff.
A little filial trust goes a long way. And I put more stock in what our current pope and bishops have to say than in how some random Catholic blogger interprets some Catholic writings (for example, on the topic of limbo that you brought up).
Which is not to say that we check our brains at the door and don’t ask questions. But at the end of the day, I’d rather be listening to the voice of the Magisterium (which is a living office) rather than those who oppose that voice.