because later episodes of the first season comes back again and again to the same siege of Cintra and its prior history, though from different angles and with different parts of the backstory being filled in. A couple of your points are explained further in a way that makes a little more sense, but most are not.

I do think Cirilla is supposed to be no older than 13 or 14, even if the actress is older (as is usually the case with actors playing teenage characters). And although it's not entirely clear, I got the impression she hasn't been the heir for very long yet, as her parents died not that long ago. But yeah, even so she does seem rather badly prepared compared to what you'd expect from a noble girl of that age and position historically speaking.