Unknown Centurio Graz museum

[Nathan Ross]

the round clipeus equester which looks oval because of the curvature of the stone...Hadrianic hairstyle.

Could be, but I don't think the tondo is deeply curved enough to cause that effect - could be 'perspective' in the relief itself, but either way it would make for a very big clipeus! I think this is more likely an oval shield of the usual auxiliary/later legionary style.

The hairstyle does look Hadrianic, but similar styles appear into the early third century. There are busts of the young Caracalla looking rather like this. All we can say is that it probably dates to somewhere within the Roman 'beard zone', cAD120-230ish...

The ring on his left ring finger indicates equestrian rank.

It's maybe possible that showing the hand raised on the sword hilt like this was designed to show off the ring. Although the Graz man doesn't appear to be wearing one... I would imagine Gessius was awarded equestrian rank by Augustus after military service in the centurionate, rather than being originally of the ordo equester - but what remains of the inscription doesn't make a point of it either way.

shoulder joints will be unable to move.

This odd 'cap sleeve' cuirass appears in several depictions, and has been discussed here before I think, to not much avail. As you say, whatever material it's made from would surely be too soft to be armour and too hard to be flexible. There's no easy answer, and it frustrates any attempts to accurately reconstruct this type of cuirass.