My question is "Will I die instantly using counterstance on Monk (or pretty much every /Mnk)?" Monk is currently a very effective tank in low man events using counterstance. It's not optimum and you can't really have any other healer but Whm. Still it gives variety to the monotony of Ninja and Paladin Tanks or just DD spamming WS.

Counterstance penalty is HUGE and it ignores several of your defense buffs like Protect. Currently with counterstance, you will take around 400-800 damage per hit from a boss monster, going up to 1-1.5k on TP attacks. The damage taken is moderate on normal monsters (x3 or x4 normal damage) are offset by the countering rate but longer fights or boss fights the only cures that would be effective on Monks are Cure IV/V spamming. Any job with less than 2k HP will have a good chance of getting killed using counterstance with the current Def/Atk vs Bosses. A single critical hit or strong TP attack will most likely put the Monk into red HP. If the Whm is busy casting something else then the Monk will have a chance of getting killed right after. If this adjustment still let counterstance get a defense penalty of anywhere near 50% (and ignores buffs like Protects, food buffs) then it will be a very useless ability that can not be used in any situation because you will either take too much damage to be cured or just get one-shotted.