Jiangmen is a prefecture-level city ("Capital" of Sze Yap). Heshan and Xinhui were actually known as "Xinhui County" but later separated into 2 counties. I've not heard of people saying "Ng Yap".
Sze Yap 4 counties = Taishan, Enping, Kaiping and Xinhui (consist of Xinhui and Heshan).
Several years ago, PRC included Si Yi (Sze Yap) into the Jiangmen District: so, now it is "Ng Yap"? I know a nice lady from Jiangmen city - she speaks Cantonese.
Guangdong ethnic groupings are complex. But I don't think that it is useful to use words like "Yue", or even "Nan-Yue". These are just generic words - similar to "Barbarian" and "Southern Barbarian" - which can be used to mean one of the many southern native groups or all the southern native groups combined.













