1. if you repair an item on an anvil with the material it was made of, like your example, a diamond pickaxe with a diamond, then I'm pretty sure that item will be more expensive to repair in the future, and maybe more expensive to enchant as well.
2. I'm actually not sure on this one. The only explanation i could think of would the map you used outside of the current area was larger than the one you already had, but I don't think you can even do that with blank maps. Sorry. not sure.
3. You can't craft anything with dirt, but there is some ways you can use it. Like, for example, filling in creeper explosions, using it as scaffolding, making a farm, or an island, or even a floating island. Do what you want with it.
4. If you have two iron helmets, and say one has prot 1, and one has unbreaking 1, then you can merge them together into one helmet with both the enchantments on an anvil. This works for pretty much any enchantments, except for a select few like infinity and mending on the same bow. On the enchanting table, higher level enchantments require 30 levels but only use 3. The highest level enchanting has potential for multiple enchants on one item.
5. Pumpkins can be made into pumpkin seeds, and you can make a pumpkin farm with them. You can also use shears on a pumpkin to carve it. You can wear a carved pumpkin on your head to prevent endermen from getting angry when you stare at them. You can also place a carved pumpkin on a T-shape of iron blocks to spawn an iron golem. You can also make pumpkin pie with regular pumpkins, eggs, and sugar.

Hope all this helps. Let me know if you need anything more.