Changes in the Roman Army

[Nathan Ross]

two distinct usages of a similar word - the 'old' Imperial Legion - and the Field Army Legio

It's the same word - Legio is the singular, Legiones the plural. Legionis is the genitive, found in the ND as, for example, praefectus legionis - 'prefect of the legion'.

So there's no difference in the titles of the different kinds of legion - the ND has both Legiones palatinae and legiones riparienses...

That there were different types is pretty uncontroversial. And if one type (the 'field legion' variety) really did have 1000-1200 men, that would seem to be a genuine innovation in late Roman unit structure...

they were no longer suitable Patrician positions.

'Patrician' means something else by the late Roman period. The earlier legates were senatorial propraetors and proconsuls - some of them were indeed patricians, but most were plebian.