Re: cog, thrust line, drag & angular momentum
The CoG of an airframe is not dependent upon trim; it's just down to the distribution of mass around the airframe and all fixed wing aircraft are designed to have their CoG at least very slightly ahead of their center of lift, more usually called the Aerodynamic center, for stability and recovery reasons (an aft CoG makes it very difficult to recover from a stall, where you've lost aerodynamic control, because it will tend to continue to promote the pitch-up attitude whereas a forward CoG will tend to pull the nose down, reducing the AoA and allowing recovery from the stall).
I can't see that where drag applies on the airframe, at least at the sub-sonic speeds that Vulture 2 is going to be achieving, is really going to be an issue.
Taking the material and size limitations of Vulture 2 into consideration, I can't really see it doing much effective gliding at very high altitudes; a controlled descent to thicker air is probably going to be the main objective immediately following launch. I think that trying to boost the speed to achieve "normal" flight at such high altitudes is going to be incompatible with the sort of high aspect-ratio wings you'd need for gliding there.
It's worth having a look at the Perlan project though, which aims to get a glider up to ~90k ft and beat the current gliding altitude record of just over 50k ft.