to certify the plane they usually need an inboard section to stall first so twist is used allowing the outboard section to always remain
above the aoa of the inboard section
winglets reduce induced drag coefficient by modifying the wing area without increasing the span [much]---this increase in aspect ratio reduces the effect of the vortices formed by the pressure differential between the upper and lower surface for a given alpha...the induced drag effects being worse at higher aoa---what effects such a modification has on the overall stall characteristic is really known only to the manufacturer so the final answer is I don't really know
