The recipe for SP and the step to making invert sugar purposely brings you in a little low. I did this because it's always easier to add more sugar than to try to take some out. You can either ferment it low or just add some regular table sugar and stir to dissolve checking SG as you add. I shoot for 1.070
As for the yeast addition, I think it's dealer's choice. If you have a healthy slurry on deck, 4 days shouldn't be a problem (if you were careful with sanitation). If you'd rather try the dry yeast addition route, you can give it a shot. I guess if you try the dry yeast and it doesn't seem to be taking as well as you like, you could follow it up with a slurry.