No need to be pissy about it....there are nearly 1000 pages of posts in this thread and your particular questions have been asked and answered repeatedly. If you had read more than "SEVERAL" pages of posts or searched the thread, you would have easily found several answers to your questions.
That said, backsweetening should be done at (or near) the end of fermentation, hence the name (after a wine has fermented out completely dry, vintners will use additives or heat to kill the yeast and then add new sugars "back in" to sweeten it....this allows for variations in how dry a particular fermentation may be and allows for adjusting to taste).
I've read every single post in this thread and while there were a couple discussions about back sweetening, there is not one that discusses adding non-fermentable sweeteners when assembling the recipe. I also asked about it and never received a response.
That was not my intention, just indicating the facts. I also found some discussion of back sweetening, but failed to find a definitive answer to add so much lactose, or so much Splenda at a certain time to achieve a level(s) of sweetness. I assume it could even be done by the bottle like bottle carbing rather than batch carbing. It may well be in here, but the sheer number of posts (even when using search) makes it a daunting task. This thread almost needs a locked sticky on procedures alone, in addition to Ed's excellent original post. IMHO.