A thought regarding wing flex.
I would be thinking of using both a forward spar and one aft as well for your wings. I would make them up with unidirectional carbon fiber spar caps top and bottom of the spars separated with a balsa shear web that is reinforced with omnidirectional carbon fiber tissue (or veil) on both sides of the webbing. With the dual spars on the wings using the carbon fiber as the spar caps should give you a very stiff wing.

As far as battery performance v temperature, is they any data from the previous flights that you have access to, that would help in determining your needed battery capacity, if not do some testing.