OP, you shouldve left this thread for me to create. If people knew the composition of chakra they'd know that illness affects it.
But this is what ill say.
Sssuke mastered the fireball technique at 7. In which Fugaki (sp) said itachi had mastered at an earlier age than sasuke. Obviously we dont know the exact age, but he mastered it alot younger than sasuke.
Now sasuke at 12 demonstrated this during his first meeting with kakashi and kakashi noted that a genin shouldnt have this much chakra for such a technique. If thats the case, id like to see his reaction to itachi demonstrating this at roughly 6 years old since thats around the time his father hinted that he was able to use it.
Now he activated the sharigan at 8. Obviously since he is an uchiha it would have little effect on his stamina, but it still did.
Now if you go back to the fight between danzo and sasuke, he talked both about amaterasu and tsukuyomi then said sasuke is indeed itachi's brother. This clearly hints that danzo has seen itachi use these techniques before. Now itachi slaughtered his clan at 12/13 and this is roughly the same time he activated the ms. If a 12/13 can use ms techniques that we all know takes up quiet a high chunk of chakra, itachi's chakra as a kid was roughly above normal for his age group. Sasuke commented after using susano v1 stage for the first time at 16/17, said that he cannot imagine how it felt for itachi, and itachi was alot younger than sasuke when he activated the ms.
Now this kimmamro issue is flawed. Just because he has a 4.5/5 for stamina can hint to many things. Firstly he has the cm2, that coudlve influenced his stamina. Next he was supposed to be oro's vessel, if thats the case he wouldve gone through abit of experiments no different to sasuke which couldve increased his stamina. Also note that 4.5/5 doesnt mean this was his original stamina. He couldve gained a 5/5, but his illness influenced this. Also dont not say an illness wouldnt have a 0.5 disadvantage, because that is also flawed, because he couldve had higher than a 5/5 for stamina but the max is capped at 5. Example sasori, hidan, kisame all have a 5/5 for stamina. If we were to estimate nagato would also have a 5/5 for stamina.
Now in the manga its already been established by nagato talking through deva path that kisame has the highest chakra in akatsuki. Therefore just because sasori and hidan also have a 5/5 for stamina doesnt mean they rival kisame, but rather the stat is capped at 5