The majority of the stress from the servos will be on the plywood, and since it is large enough, would be spread out across the whole piece so by the time it reached the screws there would be very little stress on them. You could replace them with spruce, but you are adding weight to the wing too, especially if you plan to go the twin Aileron servo setup. You have to consider then lateral balancing too, so you will want the parts to be as close to each other for both sides.
As far as the center link goes, don't know yet, haven't gotten that far in the build, it isn't until you reach the end of the manual when you attach the rod ends. My wing is sheeted, so if I screwed up, too late now.