Is there any difference between Bore dydd Sul and Bore Sul?
None at all, they are both acceptable. Personally I prefer 'bore dydd Sul' but I'm sure people speaking other dialects would choose 'bore Sul'.
Dont understand why 'Bore Sul' is correct, but 'Nos Sul' is incorrect?
Sunday morning, and Sunday night?
I had that very same question. Based on the information in this thread, however, it looks like it's just an error, so I'll be reporting it next time I find it, though it doesn't seem to be a consistent error throughout the questionnaire. -Oct 26 '17
Can you also omit dydd with other days of the week? Bore Mercher? Does that work?
Yes, you can omit 'dydd' for morning and afternoon
eg Bore Llun, Prynhawn Iau
and of course night/evening never has dydd in it
Nos Iau = Thursday night/evening