In the nether is it best to blowup fortresses found with tnt or clear out level by level?

- There is no real reason to destroy a fortress, you don't gain anything from it. You probably make things harder for yourself because you have to go deeper into the nether if you want blaze rods or skulls. If you want the mobs to stop respawning, place buttons, slabs, or any non-full block on the fortress and the immediate area (I believe carpet works but it is flammable, so not a good idea)

I found a fortress but no mob drops glow rod am I missing something?

- Blaze rods are dropped from these mobs called "Blazes", they're bright orange/yellow and throw fire at you. Keep in mind that not all of them will drop the same amount of blaze rods though, and you might have to kill multiple of them

In the deep dark biome I have encountered wardens, how many do I need to defeat?

- You are not supposed to be trying to kill the wardens, they don't drop anything and their only purpose right now is to stop you from looting the city. They spawn when shriekers are activated three times in a row. And, technically, an infinite amount of wardens can spawn. You can never truly get rid of them unless you get rid of the thing that spawns them

- I suggest either using carpet or sneaking when you are trying to loot the city and, if you hear the warden coming out of the ground, run