If you'd used more honey, it'd be sweeter. 3lbs per gallon is typically when you start producing sweeter meads, but it depends on the yeast used too (champaign yeast usually makes dry mead no matter how much honey you add).
I've never heard of back sweetening with Sprite... and would advise against it.
splenda would work though, since its non-fermentable.
it can sit on the lees awhile...after the short 7 day primary, you probably should have left it in there at least another week. I think you pulled it out of primary too early and stunted your fermentation a bit.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10