Let's jump right in with Tom's trio of questions...
We know what happens when a human and orc cross-breed, but what about elf/human intermingling? Is it possible for the other races on Azeroth to breed and are there any such results of that union?
Elf/Human crossbreeds are definitely supported by lore, as well as a whole mess of other combinations. Arator the Redeemer in Hellfire Peninsula is a Human/Elf Hybrid, for example, and there are plenty more around. Other combinations are valid as well. Rexxar is an Orc/Ogre Hybrid. Gross, I know, but oh well. There are a lot more examples you can add to those that are scattered around the world(s).
There are no hard rules in place dictating who can and cannot interbreed, but it's pretty safe to assume the races similar to one another are compatible. I would bet Dwarves and Gnomes can shack up and spawn, for example, but probably not a Gnome and a Tauren. That's just me talking, though. I could be wrong. Let's pray very, very hard that I'm not wrong.