In fairness, the scorpions could have been deployed directly in front of Carthaginian infantry. If anyone threatens the scorpions, the crews fall back, the infantry move forwards, and they're protected. This would still prevent the scorpions from firing much of the time, though.
Completely agree with all you said, but given that the battle is just three short phrases, it gives the idea that the Romans just sat there getting shot at. Two manipulus, in each flank, could had easily been ordered to attack the Scorpions forcing the crews to stop firing or keep firing and end up dead.
Also relating to the casualties, at Lake Trasimene, where the Romans were ambushed and were with their backs to a lake, the casualties were, according to Livy, only half of the Roman forces and no one can deny that it was one of Hannibal's greatest battles, so this huge casualty rate here indicates, at least in my view, that the Romans actually just stood there getting shot at the flanks, while being systematically butchered from the center.