Lactose adds way too much in the way of mouth feel and body imo. The best non-fermentable sugars you're likely to get are malt leftovers (duh) and xylitol. Xylitol isn't great, but it's better than Lactose and Aspartame which just taste awful imo. It's expensive though. This is the easiest way...
If you keg, then backsweetening isn't much of an issue. You need to add K-Sorbate and err, something else - I forget what - to stop yeast reproducing, and even then it's not "dead" and there's a chance of it restarting fermentation, but I've never had an issue with it. Then if you force carb, that should be all good, just make sure it's stirred in properly.
As for bottling, you'll need to bottle it with the sugar and all, then start cracking bottles after a week or two to see how the carb is going. Once it's ready, you'll need to pasteurize the bottles - there's a thread in the cider forum. It's time consuming but it works a treat.
Of course, you could just make a brown sugar solution, keep it in the fridge and add it to the glass when you're pouring. Not great but, meh.