Yamette_Kutasai

Not only 2 elements, but they could control more than 2 like Onoki's Kekkei Totta and some could control natural energy like Jugo. As for switching, I don't think that's how it works, because Sharingan can't copy any blood limit ninjutsu. If it's as simple as switching chakra elements or using them at the same time, Sharingan could still copy it.

I think it was from the stats from Naruto game.

As for Mei's, that's because she's also a genius in chakra transformation, beside her own blood limit. Hiruzen doesn't have much of advantage in his Sarutobi bloodline beside above average chakra capacity, but he's able to use every element to at least elite Jonin level, despite only having earth affinity. That's why people called him 'the professor of ninjutsu'.

Because Blood limit is in the same category as Kimimaro's Shikotsumyaku, Jugo's Cursed transformation, Uchiha's Sharingan, and Hyuga's Byakugan. They have a special gene which converts their neutral chakra into their desired affinity/elements, special substance, or unique abilities. They are able to control it with instinct and hone it with training. Just like how Pain able to adapt to Rinnegan without anyone teaching him.

Not exactly. Despite the chakra's affinity toward a certain element, chakra's innate and stable form are always neutral. Like natural energy. This is why ninjutsu and basic chakra manipulation are universal and easier to learn than chakra transformation. Just because you have fire affinity, doesn't mean your chakra acts like fire. Fire affinity means you have an easier time to transform your neutral chakra to fire chakra.

If you haven't noticed, everyone in Naruto has a unique chakra signature. Lorewise, it should be caused by emotion, elemental affinity, soul, etc. So, it's not impossible for every drop of chakra contains everything of what makes you unique. If your chakra contains 33% fire, 33% water, and 34% other unknown factors which the paper can't detect (Whether it is because the other affinity is too weak or inability of the paper), it is not impossible for the chakra paper to have two elemental reactions.

Beside water, he's also pretty good at earth ninjutsu as well. Obviously, you can't trust and rely on chakra paper 100%. Especially since it can't even detect yin and yang affinity, the most broken elements in narutoverse.

Having no water affinity doesn't mean you can't use water ninjutsu. In fact, every Jonin could use every basic element to a certain extent for utility purposes. It's just extremely inefficient in both training time and chakra consumption. Kakashi is somewhat an exception since he used Sharingan to learn the jutsu, but he still has the chakra penalty.

Unfortunately, he doesn't live in the middle east xD

Wait, I didn't know they made an Hanime out of that H doujin. One of the best imo.