the 10th legion Fretensis–Antoniniana in Jerusalem

[Renatus]

whoever is his source may have been misled by RIB 814 (Maryport)

Perhaps, although the theory also seems to involve Ireland and Abruzzo, so I think it's rather more developed than that!

I'm not sure about that. The title 'Fretensis' is very specific and must have come from somewhere. I think that the 'Irish legionaries' are pure fantasy and the whole thing seems to be an attempt to explain why some inhabitants of the Abruzzo region have physical characteristics more appropriate to northern Europe than Italy.

Cornelianus is an interesting figure though - I came across him before when I was looking at officers who might have participated in the Bar Kokhba war (I suspect he didn't - the cohort command would seem more likely after the centurion position, I'd say, although we do have at least one case of a tribune switching to become a centurion, so who knows...)

As I indicated, the current theory is that, after going from Britain to Judaea, Severus summoned Cornelianus, who was then prefect of Coh I Hispanorum, to fill a vacancy for a centurion in Leg X Fretensis.

Is that U shape a symbol for 'Milliaria' then?

The illustration that you posted is not entirely accurate. The photograph in the Maryport volume that I cited shows a very pronounced 'u' shape, much less 'o'-like than in your illustration or that in RIB. There is also the subscript line which is omitted from your illustration. It has been suggested that the symbol resembles the Greek letter mu and thus could stand for miliaria but I don't see that myself. David Breeze, in his article, treats it as an 'o', possibly indicating that the cohort had been reduced to half-milliary size, the symbol for a milliary unit being 'oo'. This could account for its commander being a prefect, as opposed to the tribune who would normally command a full milliary unit.

And how 'Hispan[anorum] fits in the following line I don't know!

I don't think that is a problem. The title has been abbreviated to 'Hisp', as per the transcription, or (more probably to my eye) 'His'.