Ok, I'm gonna do a brief rundown now. First of all, I'm gonna come right off and tell you that I do NOT agree that Deidara is superior. I shall explain now.
Your first point has to do with Deidara's bombs being so versitile and fast, that they can get close to Sasori. Obviously this isn't a main point, but it seems as though you think this would put Deidara on the offensive. However, the Iron sand is certainly fast enough to catch most of Deidara's bombs, and the only ones that can get through are the small ones whose explosions would NOT hurt Sasori thanks to his iron sand.
Your second point regards Deidara's C2 dragon. You say that it would go beyond Sasori's range. But that is incorrect. Firstly, there is NOTHING that Deidara can do to stop Sasori closing that range without retreating, which actually means Sasori would be on the offensive. Secondly, the amount of Iron Sand available to Sasori can surely cover such a distance. Also, Sasori can just use some Iron sand to levitate him away.
Of course, you mention clay mines. And YES those WOULD help Deidara to keep Sasori from closing the range. HOWEVER you do not not the fact that TOBI was the one who planted those mines. It was clearly a strategy that they came up with together to take advantage of Tobi's ability. In reality, Deidara cannot use this, because he needs someone else to plant the mines. So there is nothing stopping Sasori from attacking.
Your next point about C4 is correct.
Now about the suicide bomb, there is more to the scenario than you describe. He would only use this if he were poisoned. However, Sasori's poison immediately numbs and immobilizes the target, so that it can hardly move. If Deidara were to reach for more clay, or even to undo the stitches on his chest, he would do so slowly and open himself up to attacks and imminent death before he can blow himself up. Also, correct me if I'm wrong, but can't Sasori's poison disrupt chakra flow? If so, Deidara may not even be able to concentrate his chakra to the point where it would explode, let alone make such a big blast. The poison itself would aid in weakening or preventing the suicide bomb.
Deidara's C3 technique is very powerful, however Iron Sand can certainly provide good protection from it. Not only that, but it doesn't seem to me to be a projectile. It has to be dropped on the target, and so it is easy to avoid if one is airborn, which Sasori would be through his iron sand to keep up with Deidara. Similarly, that technique would only be used if Sasori was far far away from Deidara. And Deidara would most likely be unable to escape Sasori that long, and if he does, Sasori is on the offensive. That is the most dangerous scenario, because with Sasori on the offensive, and with Deidara having no substantial defensive techniques, Sasori would certainly be able to cut Deidara due to the superior speed of his puppets.
Now to the demagnetization of Sasori's iron sand. Sure, extreme heat CAN do this. It does have to be around 750 degrees Celcius however, which is pretty hot. We cannot be sure of the temperature that Deidara's explosions can reach. They may be more pressure based than fire based. We know they have flames, but we cannot guarantee that they are hot enough to demagnetize the sand. Also, we haven't even seen the explosions set fire to anything, so their burning abilities may not be so powerful. And finally, they are truly very very fast. Demagnetization would come about from sustained heating, not a short blast.
These are some ways to counter Deidara's techniques. This is frankly not my best work in terms of forming a counter, but whatever. I'll add to it and be more concise later. But right now I'll explain why Sasori's attacks would work better anyways.
Sasori's attacks are all far faster than those of Deidara. His sand is faster, his puppets are faster. Also, Sasori's attacks are overwhelming as opposed to Deidara's. Deidara's come from 1 direction, and you can see them coming and do somthing to avoid them. Sasori's attacks, either by using Iron Sand or his 100 puppets, can and WILL come from all angles, especially if Deidara is airborn. Deidara cannot, as far as we know, expel bombs to cover all of his openings. As a result, 1 miss, which is probable, would lead to 1 cut, which is deadly.
Frankly this is a weak response, but it will do. Sorry it's so late, I've been busy.