Saint Patrick & Names along the Antonine wall

[MonsGraupius]

Two names are given. one Nemthur (Not Nemthus as I wrongly wrote) which is given in the The Hymn of Fiacc, which tells us expressly that “Patrick was born at Nemthur

So this is a later Irish source, I think.

We are told there were seven forts along the wall. If we assume Medio-Nemeton was the combination of two names, Nemeton is  then the seventh at Old Kilpatrick.

Who tells us there were seven forts on the Antonine Wall?

There is rather helpful note to Nennius' History Of The Britons regarding the Antonine wall:
Caritus postea imperator reedificavit et vii. castellis munivit inter utraque ostra. (CCCC MS 139 f. 169 v)
After [emporer Severus] the emperor Carausius rebuilt [the Antonine wall] and fortified seven castles.
We know its the Antonine wall as it refers to places like Kinneil.

The standard source for Nennius' Historia Brittonum is Th. Mommsen's Monumenta Germaniae Historica Vol. 13: Chronica Minora Vol. 3 (Berlin 1898), pp. 111-222. (He consulted your Cambridge manuscript amongst other manuscripts.) You can find the received text of Paragraph 23 on p. 165, for what it's worth. I think David Dumville may have produced a modern edition in the 1980s, but I'm not sure. (And I'm not sure why you would want to consult a confusing ninth-century source for fourth-century Roman history …)

The text actually reads: Carutius postea imperator reedificavit et VII castellis munivit inter utraque ostia domumque rotundam politis lapidibus super ripam fluminis Carun, quod a suo nomine nomen accepit, fornicem in victoriae memoriam erigens construxit.
It is conjectured that "Carutius" must be Carausius. But the rest is nonsense, so why try and identify a known personage?

The main point is to ask why we should credit this version of a seven-fort Antonine Wall when it's perfectly clear that the writer is hopelessly confused.

(I was typing this while Nathan was replying. Apologies if we've crossed over anywhere.)

So this "hopelessly confused" person goes on to talk about Arthur's O'en, a Roman temple on the River Carron where there are "VII castellis munivit". However, you read this, there were seven fortified castles and the writer of the note is clearly referring to the Antonine wall.

 I'm struggling to see the relevance of the extra text except to confirm that it refers to the Antonine wall - and you don't dispute that it refers to seven castles. Is your argument: "we should ignore any texts where we think there might be a mistake" - if so we may as well give up on Roman material.