So the Norse-English mix as a substantial cause of the reduction in declension is no longer prominent?
While I can see an argument for case reduction as a result of contact with Brittonic I don't see that being the sole cause of the case reduction. A two whammy effect of Brittonic and Norse seems a more reasonable view to me.
Understandable.
A big question would be are these present in preDanelaw Northumbrian, Mercian, etc?
The Norse-only model is still prominent in general among
English anglicists, though it is more asserted than studied. But it doesn't satisfactorily explain why the "Northern Simplification Package" didn't occur in East Anglia and the East Midlands. The hypothesis that the Celtic Hypothesis school work on is that pre-Norse Western Mercian, Northumbrian and Wessex elites spoke varieties partially affected by Brittonic, with it-clefting, slightly simplified inflections and declensions, the Northern Subject Rule and other features attested in OE. Meanwhile, the peasantry of these areas was currently or recently bilingual and spoke OE varieties which already contained the much more substantial Celtic influence found in Middle English. In Northumbria, the Norse domination replaced most of the indigenous elite, and the process of contact between the 3 varieties resulted in a strong simplification outcome, whereas in East Anglia, where conditions were identical, except for the absence of Brittonic influence, no simplification occurred. This is why the Celtic innovations can be seen to spread from the North, with some never making it to standard English.
Meanwhile, in the Southwest, the standard Old English spoken by the elite continued to be hegemonic until its disappearance post-conquest unleashed a seemingly instantaneous wave of innovations, which were actually features used by non-elites much earlier. Some of these then spread into standard English, though again, others were restricted to Southwestern dialects. We can see from the Peterborough chronicle's sudden change of language that by 1066 literary Old English was already very different from what people actually spoke, so these models make a certain amount of sense, especially in light of copious historical evidence for contact.