I didn't look at any of the other artificial sweeteners, because I'd heard the best things about Splenda I guess. The others I suspect will taste too artificial.
The lactose will be in the tablets for the same reason it's in most medications: to help bind the tablet together. I'd imagine the quantity would be pretty small, as a single little tablet is equivalent to a teaspoon of sugar.
When I bottle my cider (it's been in primary three weeks, so soon), I'll do half with lactose+dextrose, the other half with just dextrose. The housemate's going to have to be content with dry cider.