1. no. Size matters, not type so much. a large immersion chiller will chill faster than a small counterflow chiller. I use immersion chillers for 30 gallon batches.
2. Unless you rack from the receiving vessel to another fermenter, after allowing it to settle out.
3. yes. an immersion chiller is sanitized by putting in your pot. takes 2 seconds. CFC is more complicated by design, although benbradford does have a point, you can just pump boiling water through it and this will probably get you pretty close to sanitized.
4. a small amount. If your cfc is constructed correctly it will probably be negligible.
5. opinions will differ. You can use a CFC to do the same thing, just put the output back in the pot at an angle, but flow will be limited by the length of the CFC. I use an immersion chiller (two, actually) and recirculate wort to create a whirlpool as you describe. It works pretty well despite our warm tap water.