Why does it not start with "maen" since "ceri lingo a dewi lingo" is a specific way of saying "nhw" which calls for "maen"?
Only the word nhw calls for maen.
If you have a plural noun, or multiple nouns, as the subject, then you use mae just like with a singular noun subject.
Why does the 'yn' not contract here? It does in the sentence 'Mae Ceri Lingo'n bert' tested earlier.