My understanding is that the perception of sweetness is the trigger for releasing insulin. That causes a "spike" in that your blood now has more than steady-state levels of it. Sugar and natural sweeteners do this too, but in those cases the insulin has something useful to do.

Depends on the carbs. Complex ones take time to metabolize into the simple ones that insulin deals with, so there's a period where, again, it doesn't have much good to do.