Let me answer at least one part of the question, on presence of Roman forces in early 5th century Britain.
Before that, however, my recommendation is not to use the Notitia Dignitatum for anything other than the most simple/basic purpose. Certainly not as an entry point to read on Western Roman Empire. This document lacks context, and is very open to too many interpretations, as you seem to be aware - your references in the comments.
Back to my answer, there was a large contingent of late Roman cavalry, the Equites Taifali in Lincolnshire (northeastern/east England) during this time period, late-4th / early-5th century.
So, even after the Sack of Rome (410CE), a significant group of Roman cavalry still remained in Britain. So, in this sense, the Notitia Dignitatum was, if you like, not incorrect.
The Equites Taifali seemed to be particularly strong fighters, and they did stay on as opposed to “returning” to Rome or Ravenna. The commas because Taifali were frontier forces, not originally Roman but most likely Central Asian in origin. Some believe they came all the way from Central Asia under the banner of Samartians. In any case, they “kept the peace” for at least about another century in Lincolnshire.
Dr Caitlin Green has the details, her own site:
As to who these cavalry troops potentially billeted and losing spurs at Ludford might have been, one reasonable possibility is that they were members of the very late Roman Equites Taifali. This cavalry unit was probably established between 395 and 398 from the Taifali of northern Italy and Gaul and is known to have been in Britain under the command of the Comes Britanniarum ('Count of the Britains') in the very late fourth to early fifth centuries. Perhaps most significantly, however, it just so happens that a neighbouring parish to Ludford, Tealby, actually bears an originally Old English name that almost certainly derives from the continental tribal-name Taifali and means '(the settlement of) the Taifali', to which the Old Norse for village, -bȳ, was added in the Anglo-Scandinavian period (Tealby < Tavelesbi/Teflesbi < Old English *Tāflas/*Tǣflas + Old Norse bȳ, with *Tāflas/*Tǣflas being the Old English form of the tribal-name Taifali). Needless to say, such a coincidence is highly suggestive, and it has furthermore been argued that the presence of this tribal-name in Lincolnshire is difficult to explain in a convincing manner without recourse to the Equites Taifali.
NOTE: See that quote in full on her site, as the details from the hyperlinks are useful. A brilliant site for late antiquity history in Britain (and outside too).