1. Lava is too an element, it is just a sub-element. Based on what you said, then earth, fire, water, and air are not elements either since they are just "the art to manipulate whatever the element is".1. Lavabending isn't an Element. It is the art to manipulate Lava.
2. Lava isn't an Element.
3. Lava isn't fluid like Water.
Your argument reply is invalid.
2. Same response as number 1.
3. Lava is fluid like water, just not naturally as fast, but can be just as fast as seen by the lavabenders on the show.
If anything, it is your argument that is invalid.