I don't think the rear shear webs are necessary. Isn't the trailing edge getting glued to the rear spars? That in effect acts as a shear web.
They don't add a lot of weight; I'd still use them even if using thin balsa. I always figured the advantage is to create rigidity of the noodle spars - thus reducing the flex in the wing at a very small weight penalty. I've seen them used more where the wing center section area and once they get closer to the tip they end up removing the aft shear webs (where there is no sheeting) and then eliminating them all together in the last few rib bays.
MY thoughts on shear webs. Yes glue them to the sides. If fitted between the spars its just a butt joint then and will add almost zero extra strength. As for ones on the rear spar. No need to do them full length of the wing but I would run them from the root rib out to the rib your wing tube terminates at. Speaking of the wing tube be sure to tie it into you shear webbing. A fillet of thickened epoxy, Fitting a piece of tri-stock, or even just some foaming gorilla glue. This will add a tremendous amount of strength in that area.
The wing is 10.6% fully symmetrical.What percentage of the thickness of the wing?