Cumnock Castle was looking out over the loch so there would be no confluence of rivers there. The only two rivers would surely be the Connel and what we now know as the Nith. Both of which had their sources near Enoch Hill, perhaps it is the confluence of Enochs' Rivers.
Hello Geoff, My suggestion that the name Cumnock is Gaelic, commun achadh 'place of the confluence', of course assumes that the name was not coined until Gaelic speaking settlers either worked or settled in New Cumnock, at the beginning of the 11th century. At this time many of the other Gaelic 'achadh' names came into use. e.g Auchincross, Auchingee, Beoch (beith achadh). I believe this would be well after the time of the great loch of New Cumnock.
Of course there are much older place-names in the parish but it is not always the oldest name that endures as the parish name. Auchinleck is a great example, Lugar Water (in the parish of Auchinleck) is an ancient name of Welsh, Brythonic origin whereas Auchinleck is clearly another Gaelic achadh name 'field of the grave-stone'.
As for Cumnock Castle of post Norman era, then it certainly would have overlooked the confluence of the Nith and the Afton, the road from Corsencon to the Castle forded the Nith and the Afton.
As for the place-name Enoch, I am not convinced this is a personal name. It is a fairly common place-name and Enoch hill is likely to be from G. eanach 'moor'. rather than name after St. Enoch.