You definitely don't have to surrender your US license when you get a UK license.
Nor should you even consider sending it to DVLA as identification if you ever plan on seeing it again. There is no legal requirement to surrender your U.S. license, but DVLA has a nasty habit of destroying licenses which it has no authority to keep in the first place.
My international friends have all needed an international driver's license to drive in the US, so I'm pretty sure you can't drive in the US with only a UK license.
It would depend where they came from. British-licensed drivers certainly do not need one.
International Driving Permits are required in both the U.S. and the U.K. of drivers who hold licenses from certain countries, but as far as U.K. license holders driving in the U.S. and U.S. license holders driving in the U.K. are concerned, they are completely unnecessary and a waste of money.
The UK licence in the US - I've hired cars in the US just using my UK licence, so for visitors thats fine....not sure if I moved there though.
No different, subject to having to get a local license within the time frame stipulated by the state in question (typically 30 to 90 days), the same as for anyone moving interstate.
I don't know about the provisional license thing... I was under the impression that I wouldn't need to get a provisional license at all (could just apply for a full UK license without one before my year is up), but after reading the boards here I'm not so sure.
You will not even be able to take the tests without a U.K. provisional license (current cost £50).
If you do get a provisional license while you can still drive on your US license, I wouldn't think you'd need L-plates or to have another driver in the car...
No, for the 12-month grace period you are treated exactly the same as anyone who holds a normal U.K. license, so no need for "L" plates, no need for an accompanying driver, and you may drive on motorways. But as soon as the 12-month period expires, if you have not passed the U.K. tests you then revert to exactly the same status as a British learner - Must drive with "L" plates, have a U.K. licensed driver accompanying you who is at least 21 and has held a license for 3 or more years, and not allowed to drive on motorways.