Huuuuuuum ...
I do have to say it seems almost as if succeeding against Tuan is about as easy as breaking titanium plates with your front teeth. While I see there are ways for Tuan to fail, as you address in your response to Rockman, that's just it. It's more ways for Tuan to fail than it is for you to succeed. With counterplay, the notion is that there are roughly as many ways for him to screw it up for you to capitalize on as there are for you to outdo him even if he does it right. Moreover, and do not take it the wrong way, but I feel one or two of your answers to him don't quite apply.
For example, let's consider the case of Teemo. The ways Teemo can screw himself over include overextending when he has no means of getting away, throwing away his only non-trap ability against the wrong target, and placing shrooms suboptimally. Ways for you to win a fight against him even when he's not messing it up include relying on your abilities to fight back while Blinded, saving your cc for when you have a real clear shot since his only means of reliably dodging are Flash and fancy feet, and carrying pinks or sweepers to periodically clear traps or lay down while fighting him to disable his shrooms.
In another example, consider Azir. The ways Azir can mess it up major include placing his soldiers everywhere but where they would actually be useful, putting himself into his own grave with misdirected dashes, and throwing his ult in a completely useless or even enemy-aiding way. As we all know, though, a good Azir can make you feel like you're playing a game you should have never even thought yourself qualified to handle because he's gonna dance all the hell over the place, carve you five new ass holes, and look good doing it. That said, you can still outplay him, even if he knows what he's doing. Some ways to do so include feinting a fight to make him throw soldiers in the wrong place before moving the fight away from his cronies, closing on him directly after he dashes and forcing him to gtfo, and saving all your most salt-creating cc for that moment the cocky prick comes flying in looking for a Shurima Shuffle.
With Tuan, here, you can win if he messes up. Tuan can mess up by wasting his abilities, which have rather long cooldowns. With cdr the way it is though, those cooldowns can become far less so with the right build, and there are a wide variety of items that give cdr. If Tuan builds tanky enough, then he can arguably be alive for long enough to use his abilities again anyway in the same fight. As far as outplaying him goes, you can peck him once during his Q to make it so it doesn't hurt, but that's assuming you notice it's activated in the meteor shower of a LoL teamfight where you are most likely looking to peel you carry/dive the backline/disrupt. If Tuan casually walks into the melee with 90% increased tank stats of course you're more likely to ignore him than anything, which means you'll eat a hot spray of damage for having pointed the guns elsewhere. Except that you told Rockman that killing him before his Q went off was still an option. So that means players would throw hot burst in the tank's direction which probably won't kill him anyway if he starts the channel and gets tankier. And no player that would be able to burst Tuan down should be hitting him anyway. Burst mages and glass cannons have better targets than Tuan. It's actually considered a dumb move to blow the big stuff on the tank if you can help it. Talon might have the means to kill Tuan before the channel starts, but Talon should be going around him and killing the adc, especially since Tuan's passive is a soft Thornmail and Talon would just be killing himself to try assassinating Tuan.
On the off chance everyone backs up and Tuan's Q doesn't do anything at all, it still would be stupid to try killing him even if he is alone. Everyone hitting him takes damage back for doing so, and one of them is probably gonna get splattered across the lane by Tuan's death animation if the second part of his passive is off cooldown. It effectively means that trying to answer his push is a waste of time for everyone. Even if you do kill him and his passive doesn't go off, you'll likely be wounded enough from his passive anyway that you'll need to back. His passive, Q, and E mean that honestly you should never acknowledge him. It'd be like being the adc and pumping rounds into a Thornmail Rammus. Unlike a Thornmail Rammus, who can be dealt with by an AP teammate in your stead, Tuan screws up both damage types. In a teamfight, all you can hope to do is cc him before he gets into the fight properly and keep him there while you try to have a fair fight with the rest of his team.
While his W is interesting, it would be just as interesting and helpful if it affected one target and not all targets it happens to hit. This means if he flashes in or something and hits the whole enemy team with it, he can turn on his Q and take everything. Even if it kills him, just having been hit will have dealt a lot of damage back against the enemy team (maybe killing someone if his passive is up). And if it doesn't kill him, he's still close enough for that Q, fueled by all those attacks, to wipe the team. And in case you wanted to get away to keep that Q from catching you after you accidentally pumped a Bio-Arcane Barrage and three Living Artilleries into it, his E can keep you there. And even if you survive this one man army's opening gambit, he can have his Big Brother Ult chase your sorry ass down.
This all of course is without factoring the issue that there are four other people also doing any number of other things to blow you to pieces. You have to put up with Tuan's Tomfoolery while his midlaner throws their spell combo in, his jungler picks someone to rough up, his adc throws crits into your team, and his support makes it expressly more difficult to kill Tuan than it already was. Tuan can't be ignored because he's too disruptive and too damaging. Dr. Mundo can often be ignored because he's not disruptive. If you're an adc and the enemy team's frontline is a Dr. Mundo, just walk around him while your team peels the guy off you and shoot someone worth shooting. If Mundo does reach you you can probably just lifesteal his damage off of his friends. Or even off of him; just turn the hapless purple quack into a health battery. That's not an option if Tuan's there because Tuan only has to reach you once to W you and absorb everything you could try to do. There are a wealth of ways for Tuan to get close enough to do that besides Flash. Speed boosts from Ghost, Whimsy, Righteous Glory, or nearly anything at all can let him bull through your teammates, who probably know better than to waste attacks on him and won't peel him anyway, long enough for a curse.
I think the notion of ability redirection is a neat one, but one that needs done very carefully. Even if it only affects one person, it can still be terribly broken if make incorrectly. His Thornmailesque passive is also interesting and could be a very neat idea, but the second part's kamikaze effect is unneeded. Other death passives require some manner of input to be useful. Sion and Kog'Maw still have to move and Karthus and old Zyra still have to aim. Aatrox, Anivia, and Zac don't have input but their death passives are revives, not parting shots. Tuan's automatically backhands someone without his input. It's all well and good to argue that you can kill him with a mild poke to waste the passive cooldown on negligible damage, but if you're trying to kill a tank there's no mild poke about it. There are a mess of execution attacks in League, and if you have one you're gonna use it, not risk him limping away on 10 HP because he chugged a potion and your raw-AP Fiddlesticks auto didn't have enough oomph to finish him.
As interesting as Tuan's unique kit is, I'm afraid I'm with Rockman on saying it's just broken as it is. To make it salvageable, I would recommend changing the W to affect only one target, dropping or adjusting the second part of the passive, and finally removing most of the damage scalings on his abilities. Tanks are disruptive, and to be allowed to have that kind of game-changing cc they have to do less damage. Champions like Maokai and Nautilus don't do worthwhile damage unless they give up on being tanks, and if they do that they become as squishy as, say, a Brand or a Ryze. While it is true some tanks still have damage, that is because their cc is of a different value. Tuan's "shoot anywhere is still hits me" (which by the way I feel would be very nearly impossible to program anyway given the number of strange cases (if he takes Twitch's Spray and Pray bolts, should they be couted triple or something because of their potential damage? (if he redirects an AoE, does he take more than the usual damage for that same reason? (what if it's a neutral ability like Bard's Tempered Fate?)))) is simply extremely valuable. So much so that Tuan's damage should be almost exclusively tied to his passive and maybe his ult. Especially since that curse isn't even his only cc tool.
In any case, I'm sorry to have bashed it in such a fashion, but this is the way I see it. It's certainly salvageable, and the mechanic an interesting one. I'd like to see it done more properly, but as it stands it's just a little too much, I'm afraid. Of course, that is only my humble opinion.
