Re: cog, thrust line, drag & angular momentum
I don't think that there will be a problem with getting the thrust line and CoG etc. correct; I gather that the airframe is going to be computer 'printed' from powdered nylon and so will be accurate and conform to very fine tolerances.
Swept wings on a low-speed aircraft, like Vulture 2, are not a good idea; whilst swept wings bring benefits in high-speed flight, at low airspeeds the air starts to flow along the outer part of the wing instead of across it. A long, straight, high aspect-ratio wing is really what's desirable, and I trust that the soton lads will know how to arrange the internal bracing to reduce twisting.
Once again, there's no reason to suppose that the direction of thrust from the rocket motor will be so variable or that it will be mounted so badly that it will result in off-axis thrust and produce a significant turning moment. In any case, Vulture 2 will have flight control surfaces and an active autopilot/FCS.
You shouldn't be thinking in terms of combining the mass of the launch platform with the mass of the aircraft; the only binding between them will be the residual friction between the launch rod and the runners; apart from that residual friction, the force from the rocket will only be acting upon the aircraft.
(After doing some force vector diagrams and numbers I've come to the conclusion that the launch rod is the best solution; sure, before the rocket is ignited, the aircraft will be 'hanging' from the launch rod but once the rocket motor starts to burn the aircraft will actually be pushing up against the launch rod and no longer hanging from it. The wing guides will be necessary not only to prevent the aircraft from flapping about too much during any low-altitude turbulence but also to stop it from twisting around the rod due to that upwards force on the launch rod once the motor is burning).