Orsova ballista field trials to start.

[P. Clodius Secundus]

... envisaging the horsemen dismounting and wheeling out a carroballista surely isn't in keeping with the general context of cavalry manoeuvres!

You may have some difficulty explaining that to the Hussar Artillery. On this side of the pond I would refer you to "The Gallant Pelham" J.E.B. Stuart's boy wonder of the horse artillery. (one of his favorite toys was the 12lb Napoleon, designed by Verchere DeReffye BTW). As interesting an image as this conjures up, it is hampered by the lack of evidence that carroballista carts were ever ridden. I suppose they could have led the team from horseback and sidled up behind the cart to fire the weapon, but that's really stretching it.

....and I concluded that it must be a miniature catapult of some kind; a Hadrianic date suggests either the cheiroballistra itself, or the much less-well-known arcuballista, which I proposed might have been more like a standard crossbow.)....

It only suggests a cheiroballistra if you assume that Heron's design was actually adopted. It may well have been, but I've seen no objective proof. In it's absence I would fall back on the well-documented Xanten or better yet, the arcuballista which was presumably simpler to use while mounted.