If there's a high-enough iodide concentration in orange juice to produce the thick cloud of iodine fumes after a single drop of sulfuric acid, this would also make the juice taste far too acrid and bitter to drink.
The half-life of Iodine 131 is 8.1 days. If the contents of the orange juice was enough to kill Belle in 5 days, in only 1-2 days after ingesting the lethal dose there would still be enough left on the orange juice to be considered extremely dangerous, and very easily detectable with radioactivity sensors.
Every aspect of how Iodine 131 is kept and transported is nonsense. It would not be kept in plastic syringes in a standard fridge; heavy lead shielding is needed to handle it safely but we see none; the ease with which the murderer got hold of the lethal amounts of I-131 shows that the safety measures were grossly inadequate and would have attracted much attention from regulatory authorities. Further, the murderer would be subjecting himself to very large amounts of radiation by handling I-131 in the manner that is shown.
Radiactive contamination does not make organic matter, like skin, evaporate into nothingness, even when being very powerful.
Horatio states that the Iodine 131 decays to normal Iodine (isotope 127). I-131 decays via beta radiation to Xenon 131 (a neutron converts to a proton and releases an electron). Xenon 131 is stable and a noble gas (doesn't react). It would not be detectable with sulfur.
Caliegh is shown sniffing the suspected poisoned juice. Proper scientific safety protocol would be to use her hand to wave odors towards her. A veteran CSI should know to do this.