Legal nuances

I have two questions which I've raised in class, and for which I still don't really understand the answer.

The first relates to why tear gas is illegal against combatants under the chemical weapons ban. Ostensibly, it is a chemical weapon, so it should be banned. However, if the point to international humanitarian law is to make war more humane, then it seems that the international community should support the use of tear gas as one of those "less lethal" weapons. Of course, all weapons, whether declared legal or otherwise, have their risks. However, the three main teargas varieties (CS, CN, and CR), are largely non-lethal (though CR can, in unventilated areas, be lethal, and some individuals can be particularly sensitive to teargas). For the most part, teargas is not dangerous. It can subdue an enemy without permanently disabling them. Why not use it against combatants? If a military force's other option is to kill a combatant, then teargas is by far a more humane weapon.

The other question is one I brought up last class. If a military medic or chaplain picks up arms during combat for reasons other than self-defense, what becomes of their status? If captured, are they now prisoners of war, who can be held until the end of the conflict? Are they still protected persons who must be released back to to their side? Or are they unlawful combatants, who can be convicted of war crimes for perfidy? Article 24 of the First Geneva Convention states

"Medical personnel exclusively engaged in the search for, or the collection, transport or treatment of the wounded or sick, or in the prevention of disease, staff exclusively engaged in the administration of medical units and establishments, as well as chaplains attached to the armed forces, shall be respected and protected in all circumstances."

I suppose that, on the basis of the laws I was able to find, if military medical and religious personnel pick up weapons, they are no longer protected. I didn't find anything about them becoming unlawful combatants. I suspect, though, that they could later be charged with perfidy in the same way that someone illegally posing as a civilian can be charged with war crimes.