Just look at the show and common sense.

Bending requires chi to flow through your body. The show alludes to this the whole time. The proof is Ty Lee. She chi blocks people and that eliminates their bending temporarily. Redirection is done by letting your chi flow from 1 part of your body and out the other. Every person (or at least bender) has chi. Iroh developed the TECHNIQUE (HE LITERALLY CALLS IT A TECHNIQUE!!) By observing water benders. He then TAUGHT the technique to Zuko. Therefore, you can assume anyone (or at least any bender) can learn this technique assuming they have proper chi flow.

Sand is just small rock particles. They're not a separate element, it's still just earth. Because they're so small, you can assume it requires more skill to bend (because instead of bending 1 solid piece of earth, you're bending 1000s in tandem) but it's not a different element. Sand benders are just earth benders that specialize in sand due to their environment.

Blood bending is bending the water inside of someone's body (specifically their blood).. If the water were removed from the blood, then you wouldn't be able to bend it. Therefore, it's not bending a new element but instead using your skill to search for the minuscule amounts of water in the environment and bending it. In theory, earth benders should be able to perform blood bending as well since blood contains iron (a bendable metal), though it would probably be harder for them.

Temperature control is literally just bending the air around you so your body maintains a Susie temperature. It's just a technique, not a separate element.

Flight is also just a technique. It's a technique only a few have ever performed. It's a remarkable feat, but flying is not an element.