It's not the export_descr_units that determines the projectile damage to target types. There is a separate missile file which defines what damage "scorpion" missiles do to targets.
So, I changed the damage to buildings to 1 for scorpions, but the AI stacks with scorpions in them still just stand outside the walls until the timer runs out. I checked, scorpions do show 1 damage to buildings in their stat sheets.
Is there another "thing" that triggers/enables attacking the walls? I can see that AI stacks with ballista in them attack the towns just fine (same as stacks without any artillery). It's just the scorpion equipped ones that seem to mess up.
p.s. On a side note, I wish there was a way to find what causes the "next turn" crashes. My Seleucid campaign came to an abrupt halt due to the same bug, reloading the last 10 turns did not help... Is there a crash-dump somewhere for RTW? LOL, I don't remember where to look but vaguely remember there used to be such a thing.