I think we need to do a few more experiments to determine your answer to number 1. I suspect that stopping the ferment before all of the sugar is used up will yield the best flavor, however letting it ferment all of the way out is much more convenient.
Check out this awesome discussion on the brewing network that describes the same process but done commercially by Two Rivers Cider out on the west coast.
http://www.thebrewingnetwork.com/membersarchive/dwnldarchive05-21-06.mp3
I see where you are coming from Tusch, but if you let the cider ferment out all of the way, there is more alcohol in your cider so it might be ok to cut it just a little.
I just tried some backsweetening on one of my batches but I was using un-pasteurized cider. I ran everything through a rough plate filter to try to get all of the yeast out after cold crashing. I'm not sure quite yet if that got all of the yeast or if I should also put the polish mat in the filter and run it through twice. That was last week, I'll check the gravity & taste and report back.
Kevin can probably answer the temp question for #2. I just chilled everything down to about 35F.