1. The Death Star is Tarkin's home ground, his territory. Vader is the guest.
4. The garbage also has to be transported from deep inside the Death Star through tunnels to the surface, possibly through intermediate facilities as well. It is much easier if the garbage takes little space. Note that garbage trucks in the real world also have built-in trash compactors.
Also, you can't dump anything into space. Floating space debris orbiting the Death Star can be a real hazard for ships and other operations. In the real world, debris from satellite launches orbiting the Earth have already become a problem.
6. GL got it wrong, OK!
It was later explained in the EU book "Rebel Dawn" that The Millenium Falcon traveled very close to a cluster of black holes ("The Maw cluster") whose immense gravity distorted space thereby shortening the total distance. Nobody had flown that close to the Maw cluster on the Kessel run before or since, and lived.
7. Imperial "Credits" ?
8. He was built from parts of other droids.
As for technical questions: You can look them up in a Star Wars technical books or the various online encyclopedias:
http://starwars.wikia.com
http://www.starwars.com/databank/
http://www.theforce.net/swenc/