Chiasms are mostly used in poetry and high rhetoric, for dramatic or or playful effect. What they do is emphasise the words that seem inverted, draw the reader or listener's attention. I would say the frequency and effect of chiasms were not so different from how they are used in the modern languages. I think the 'C' in ABCBA could be anything; it depends ...
The Saturnian was (probably) stress-based, not weight-based.
To borrow from another answer of mine:
In a question about Old Latin meters, an anonymous user brought
up Mercado's convincing argument that the Saturnian was based on
accent. The idea isn't new, but Mercado backs it up with some nice
information-theoretical analysis: basically, the ...