Certain things have been given to us already. There is no seal to Deidara's (art) ability. The idea of how to gain the mouths in the hands by defeating Deidara via explosion was previously explained. The similarity between the arms and the sharingan (eye) Kakashi has makes complete sense. To answer the rest of your questions, the clay can come from anywhere. The explosion is caused by infusing his chakra into the clay. Like with Gaara he infuses his chakra into the sand.
Try not to over think things by saying where will he get more clay. He seems to always have enough to do the job. I mean are we starting to count how many items (explosion tags, shurikens, etc..) each ninja takes with them in the anime? It's an anime therefore the characters bring enough items with them.
1.) seriously how many deidras are there going to be?
youll have a hundred people running around with 1 deidras arms.
(mounts are different since theyre summons)
2.) we dont know if he PREVIOUSLY used the seals before using his attacks.
3.) i understand the chakra part of it but still, the problem of how hundreds (or even maybe 10 people) will manage to get them is still a problem
How many people will have bloodline abilities compared to the anime? How many people right now have the Curse seal compared to the anime? Not to even mention the Curse Seal Level 2? How many people have the seven star tattoo? Finally how many people have the kyuubi and shukaku right now compared to the anime? It's pretty obvious that only one person in the anime could summon the kyuubi and only one person could summon shukaku. Choosing which things to cater to and which one's not to cater to is really the discussion here. I believe their will be less people to have this ability than the curse seal, due to the fact that a person has to first gain access into the Hidden Rock Village. If a person has gained access into the Hidden Rock Village don't you think they deserve a chance to gain a speciality? It's not like the speciality is being handed to them on a silver platter, because of the number of dks they have gained.
People gave up a good amount to enter, then they still have to earn the right to gain the speciality.
CS and SST
we dont know how may people oro gave the CS to.
remember?
he had whole labs developed to test the results and etc.
there may have been hundred who had survived with one.
and who knows how many of those managed to gain the CS2 form.
also SST isnt that one of neji's creations? its because the evil people had too many things for them.
its not that i dont like this jutsu is just that its too many things involved thats all.
neji would have to create shops to BUY the clay, hed need to make events where you find it, hed have to make the deidra fighting system, hed have to program it to be almost impossible to beat (since it is a very strong jutsu use youll recieve) and all that other stuff.