Nice post.
Regarding the doton thing, I assume it relates to the word shape manipulation. There is where the affinity part comes in.
Because the way things were going before the training Kakashi could use 1000 jutsus, and all shared different elements.
So probably anyone can use a elemental specific technique if he knows the seals. Sasuke and any other Uchiha could use the fire breath even thou they had different afinities.
Maybe A's and the third Raikage's lightning armor is simpler to do than Sasuke's or Kakashi's chidori. All shinobi could do simple shapes or just generate the effect while only someone with affinity can shape it.
I suppose that makes sense but I still can't help but feel that Kishi is cheapening the worth of elemental ninjutsu and affinity with everyone proving capable of creating perfectly these barriers of earth without any imperfections or difficulties being implied. It's similar to the situation from P1 where Tobirama's ability to use Suiton without any nearby sources was held in high regard but now since P2 started it's par for the course of anyone who can use water elemental ninjutsu.
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Shikamaru asked Kitsuchi the handseal formula for simple doton barriers. Ninjas, from a very young age were taught how to knead chakra with handseals If you can get the formula, you can use the jutsu. The effectiveness vary from person to person, but they can still create the barriers with the basic kneading technique. What makes Shikamaru's plan more effective is that there's enough quantity of Shinobi to slow down the Bijuudama.
What you are referencing is referred to as shape transformation. Basically, it's the altering of movement and composition of one's chakra to forge it into any specific size, range, application, and momentum for a jutsu. They learned the basics of these precepts when they were at the academy.
Nature transformation, the usage of elements, is an entirely different process where one is required to convert and then mould their chakra into a natural element, thereby changing its very nature and properties. There's a reason that Naruto and Sasuke, when they were training to perfect their respective elements, were having such a difficult time in the process. It's an advanced form of chakra control which only a handful of people are capable of using to any practical degree. At least that was the implication back during P1. Even now though I don't believe that there have been many people depicted of being capable of using elemental ninjutsu who are below the rank of Jonin. The only exceptions are Sasuke (part of a clan who possesses an extremely powerful doujutsu and highly advanced combat skills), Naruto (also part of a powerful clan and he also has Kurama to provide him with chakra), Temari (considered to be incredibly gifted and is one of the best Futon users in the manga), and Omoi (who apparently only knows "a little).
I'm probably missing a few people but the general idea seems to be that elemental usage is difficult to grasp and it's mostly just the elite and gifted who are capable of using it.
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The Futon training drill Naruto went through was for effectiveness.
Really? Apart from the FRS it's very likely that Naruto has virtually no comprehension of his element or how to manipulate it into different forms excluding that small Fuuton Rasengan that he displayed when arriving to assist Kakashi against Kakuzu. Naruto's one elemental attack is very effective but that's all he has. I wouldn't exactly call him a knowledgeable or experienced user of the elements.
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But what Shikamaru's aiming here is the quantity. It doesn't matter if one's doton is weak compared to the others (as said person never went through the doton training regiment) but with the basic kneading method, they can still use and create it, disregarding the strength of the walls.
I addressed this two points above.