Update: undeploying the existing turret and destroying the ghost turret, and then demoting/re-promoting the engineer did indeed solve the issue of not being able to place the second turret.
However, the fire.json error linked here persists, and appears to be preventing the turrets from firing at all. They target hostiles (or at least they turn toward them when in range) but I see no projectiles or firing animations, nor do the hostiles appear to take damage from them.