The noise was really scary, I am not sure if it was added for dramatic effect in the trailer only. However, in the new versions of the original trilogy, the dark side of the force has sound associated with it, like when Vader uses force choke or force hurl you can hear a rumbling sound.
If you notice, the Emperor starts to physically deteriorate quickly once the "cat is out of the bag", that is, he no longer has to shield his powers so they consume his mortal frame more rapidly....
to answer your questions:
1. Memory wipe for 3PO, there are probably thousands of 3Po driods out there, hard to tell them apart.
2. Most of those who know about Luke's heritage was already dead, and with Luke trapped on a back water world like Tatooine, with Obi Wan to watch him, there was little reason to worry about his name. However, Leia's name was changed so she could remain "annonymous". Perhaps Vader knew that he was going to have a son, but he DEFINTELY didn't realize his wife would have Twins, as evident in ROTJ.
3. Ben was watching him from afar, which is why he knew Luke was in trouble with the Tusken Raiders in the first movie...