Nothing to add to the mega stones (I mean, they do one thing and one thing only, so good job criticising those lol).
About the liquid nitrogen, it might or might not be broken, can't tell as it is. Apparently it works by being hit by a foe, and then 1 out of 10 times it will freeze the attacker, right? now, does it count for all moves, or just contact ones (I'm assuming just these, but just incase...)? Remember that freeze is a broken status condition, since it doesn't end after a set amount of turn, and nothing can be done while affected by it. On the other hand, freeze is incredibly rare, generally only being a secondary effect of ice type attacks, so really dunno, this item might just work alright in the current metagame. I like it, in any case.
So shifter's gauntlet is basically a variation darmanitan's zen mode ability but in the form of an item any pokemon can use huh? well we all know zen mode darmanitan sucks because it becomes so slow after losing half its HP, and can't really take that many hits even after the defense buff, since it would be at 50% HP or less anyway. However, there ARE pokemon that would benefit greatly from this: the slow conkeldurr (or most slow and bulky fighting types) would get a HUGE attack stat without being limited to choice band (think about this for a second) for as long as they remain above half HP, and would become extra bulky once worn down to below half. Think of mons with few weaknesses, like Eelektross (none) or Rotom-wash: they get MASSIVE damage output in their respective STABS (50% boost to rotom-wash's hydro pump? uh...), and would become super bulky at half HP. Not only that, but they can become bulky enough to take a hit at half HP or less, carry rest, use rest, and be back at full HP, with attack raised again.
Broken? not sure, again. Good? hell yeah. Maybe too good, but can never know until you see it inside the system. One thing is for sure: this item would help archeops a lot.
I'm really not sure what to think about this one: with types like fire, rock, ice, electric, fighting or ground it would be super broken, since weaknesses to those are common, but steel... few things are weak to it that don't die to an unboosted iron head or bullet punch, specially ice and fairy types. I guess in most cases, if it works like stealth rock, to a mon who's neutral on steel, it would take 12.5% of its HP plus whatever damage the move actually did. You might think this is not broken because steel is not common as a weakness, but.... scizor with bullet punch..... iron head excadrill.... they hit hard, and they take 12.5% extra of your HP for no actual reason....that's a scary thought.
Amazing item for mons like Corsola, Lapras itself, Nidoqueen.... but...... blissey? skarmory? giratina? regenerator slowbro? alomomola? roost mandibuzz? like it happens often, there are mons that benefit from this item just fine and make it seem like there's no problem with it, but then there are others that become pretty unkillable, making them exceel too much.