Also the turtles stated no one has MASTER all 4 elements before, they never said anything about no one ever learning more than one.
Actually, you might want to go back and check yourself, because Raava specifically states that the human body cannot handle more than one bending at a time. Ravaa became the conducter of the bending. When she passed through the first, she was changing his bending so that he could master them all. The only reason he was able to use all 4 at the same time was when Ravaa merged with him and she expressly stated that if they stayed in that state for more than a few minutes it could kill him. Harmonic convergence fused Ravaa with the first. That is why every avatar is able to bend all 4 elements. To be able to multibend, they would have to wait for 1000 years until the next harmonic convergence. Even then, you would have to have a spirit willing to be fused with 0 control of the body. Not something many would do. The only reason Ravaa did was to save the world.
Also, mud can be either water or earth. It just depends on the concentration of dirt to water. Also, theoretically, metal benders could control blood if there was enough iron in the body. The reason Katara needed toph to clean the water wasn't because she couldn't bend the water, but because she couldn't pull the sludge out of it. Toph could bend the sludge, but couldn't get rid of the water.
Also, harmonic convergence only unlocked those with the potential to airbend, which, as toph stated in the first season, is because of how you approach a situation.
Also, if you read the background of avatar. The air nomads were the only nation that was 100% benders. Thus why only airbenders appeared after the harmonic convergence.
From what I can see, it only looks like you want two bending elements to prevent having any weaknesses.