1. The heat will drive out most residule carbonation when you mull the cider; carb it all or carb half, whatever is easier for you.
2. Backsweeten with apple concentrate or sugar, or brown sugar, or honey, or whatever else you want; this is mostly a taste pref. But be sure to watch yourself lest you end up with bombs.
3. I add spices to the glass for hot cider, and add to the bottle for cold cider, but again, I think its just a personal preference issue. Add to the secondary (if you secondary) at the soonest, to the glass at the latest :P
Have fun with it and appreciate the art of it all