1. Make a teleporter not be silent As siad the teleporter aren't silent so I don't know what you are getting at here.
2. Make an elevator with four activation switches There are a few tutorials at: http://www.claudec.com On how to make an elevator, but you should go into more detail becuase I think what you are trying to do can't be done. there is no way to make a triggered elevator stop at mutliple floors with out making a mod for it, which requires alot of scripting.
3. Make a switch-activated teleporter That wouldn't work really but you can make a trigger_multiple set a delay on it and have it activate the teleporter, but that might be alittle weird.
4. Make a spiral staircase Their are alot of tutorials on how to do this and prefabs already done I a fairly basic tutorial on how to do it. http://planetquake.com/bubba/spiral1.html there are more adavnced one here http://www.claudec.com
5. Make a zone in which there is no gravity I am not sure how to do this, but you might be able to make the all the walls, including walls and ceiling all func_push
MAN I can't believe I gave links but you are asking alot of questions that are already been covered by many many great tutorials.
Wouldn't you have to create another worldspawn and set the gravity lower in the particular room....?
Can you have more thatn one worldspawn in a map?
Not sure but I DO know that gravity is set by worldspawn.
I don't belive it is possible.