1.) No matter whether it is a armor or your body, if the hardness of your body surpasses the protection of Haki then you do not need it, at least following your logic. Besides, a physical armor like you said does not really provide protection from heat, fire, cold, at least not in the level we are talking about (Aokiji´s DF, Akainu´s magma)
I have to admit I can't follow your logic (or you just don't get mine), any additional protection is needed in my logic. Of course, normal haki armor shouldn't able to, but I wouldn't say Shanks' haki is normal. I dare say his haki is one of the strongest at the present. To get the comparison, let's just say that normal people's CoA is like common leather armors, and Shanks' is like one made of the strongest material in the world.
2.) You say that CoA is a physical armor that somehow enables you to touch the real body of DF users without giving a proper explanation to why a solely physical armor should be able to do that. The only possible solution for that would be that CoA provides you with an armor so hard and superior that you somehow hit the real body (again, under your assumption taht the armor is solely physical). Following that logic should make clear that Jozu would not need Haki because his fruit gives him such hardness superior to any physical armor CoA should provide (unless you think that CoA armor is harder than diamond which in turn would mean that Luffy actually punched through an armor like that)
sighs
Okay I'll give you my theory (one of them actually): let's not forget that the meaning of "haki" is something in line of "ambition" and "aspiration", which implies strong mental strength or willpower that manifest into something that can be useful in a fight. In case of CoA (but this is just my theory), the mental strength materialize into a layer of some kind of energy/field/aura/whatever that provides protection as well as enables "catching" DF users' real body, because it represents the will of the haki user to protect the body physically as well as the will to physically touch the opponent's body (not just DF users), both enhance the defense and offense of the user.
The way I see it, DF powers are simply the power to manipulate natural laws (again, in OP world). If haki can be used to cancel the effect of DF powers, it should be able to be used to cancel natural law too. Any disabling means should be on the user, not the effect.
3.) Again, under your assumption, CoA should enhance your physical abilities (both offense and defense). I am saying in return that Moria´s Shadow Asgard should be superior to such an enhancement and thus he should be able to touch Luffy´s real body which did not happen. Btw, we discuss in the logic and the world of One Piece, obviously the real world rules do not apply at all.
I have to say you misunderstood me. The ability to touch doesn't come from the physical enhancement (both offense and defense), rather, both the enhancement and the ability to catch DF users' body come from the same thing, which is CoA.
Really, you misunderstood me in many ways, and it seems like you're putting words into my mouth.
BB´s DF does not nullify the DF power per se, as soon as BB touches his opponent it sucks in the power itself since the Yami Yami No Mi is basically gravitation (btw, my theory on how BB was able to get WB´s power). For that though BB has to 1.) touch the opponent and 2.) know when he is going to be attacked and use his "darkness" to suck the power in, meaning that any kind of long range attack or projectile has to be perceived by BB (whether it just normal senses or CoO) so that he can spread his darkness on that point. Since Ace´s attacks were to fast for the "slow" BB it is quite logical that he could not react to it.
Personally, i would not bring Kairoseki into the discussion because except a statement from Smoker we do not how it works. For example, Robin could not use her ability while having the Kairoseki chains but Luffy was still rubber unterwater. We have to wait till the Vegapunk Arc to get a full and, more important, conclusive explanation.
I also still do not agree with the last point you make. While you are right that CoA does not drain strength or prevent the opponent from using his power, it does cancel the product of the DF (best example would be this http://www.mangareader.net/103-2619-6/one-piece/chapter-512.html, Kizaru´s body is still there but he emits light in order to travel but Rayleigh uses his CoA enforced sword to prevent the light to go further. If we would follow your logic in this example then Rayleigh should have cut Kizaru´s body thus hurting Kizaru but there was no sign of it whatsoever).
One question: Do you think BB can suck Law's room without touching Law?
I do think seastone cuffs are not made purely from seastone, just enough to rob the DF ability and physical strength but still allow the cuffed DF user to walk (in a weakened state).
Kizaru didn't move as a single beam. He transformed into little beams which path was blocked by Rayleigh. If he was moving as a single beam, he would have been cut by Rayleigh. And for this instance, the light IS Kizaru's body. If, for example, Kizaru uses his laser as a product, no matter what Rayleigh does he won't be able to cut it to injure Kizaru, since the laser is the product, not the body. And I dare say Rayleigh is not immune to Kizaru's laser.
Just like BB and Ace, any flame shot by Ace is just fire after it separates from his body. Thus, a touch to the flame doesn't cancel it.
I think your mistake is that you put the DFs into two different categories, those fruits who change your body (Logias, Luffy) and those who give you a certain ability (Hancock for example). You forget though that both, the abilities and effects of the DF and the Logias + Luffy´s rubber body are a product of the DF itself. It would not make sense that while CoA enables you to attack Logias + Luffy´s real body and ignoring their DF body and thus the DF product, you could not use CoA to cancel the effect of DF other than those that change your body.
More questions: Why Momonga needed to hurt himself then? Why didn't Smoker just use CoA against Law, why did he need to dodge?
Furthermore, although these are hypothetical: can CoA be used to remove shadows from Moriah's zombies by just touching? Can it be used to dispel Kinemon's clothing? Can it be used to return people who are stoned by Hancock back to human? I highly doubt it.
With your logic, I'm confused why you agreed with me that CoA doesn't cancel the DF power from target's body? If it can be imbued to arrows, I don't see why it can't be used to "coat" people to cancel their DF powers.
My point is, CoA can be used to catch DF users' real body, albeit the state, as long as it's the body.