Vulture 2 Suggestions
Don't want to step on any toes, but thought some of my experiences may help.
I thought that I may be able to help with suggestions, from an engineer and pilot with high-power rocket experience. I have successfully flown a two stage "L" powered rocket with radio second stage ignition and radio chute ejection.
Years ago, motor ignition was a constant problem. The solution was to use a reliable electric match (or two) and epoxy it into a plug (chunk) of actual propellant. The propellant will not ignite while cutting with a sharp knife. Cut a piece about 1" long and trim the sides to fit into the motor nozzle. Use a drill bit, in your hand, (no drill needed) to make a hole for the electric match. Use some quick set epoxy to glue the match inside the hole. When ready, epoxy the whole assembly inside the top end of the rocket motor. When that electric match is fired, that motor WILL burn. Never had a failure that way.
Igniters that pop, like a firecracker, will usually not light a motor.
If you are worried about the cold, you can test fire it after being wrapped in dry ice for an hour.
(disregard. it seems you have an altitude chamber)
1 - These motor casings are quite hot after a burn. Even though it is cold up high, can the motor soften or melt the fuselage?
2 - As mentioned in a previous post, stability in the thin air may be a problem. Basic rocketry tells us that the aerodynamic center of pressure (CP) must be behind the center of gravity (CG). The more the better. You have a unique situation. There isn't much aerodynamic anything at 100,000', so, until you build a lot of speed, you will rely on the center of thrust being aligned with the center of gravity. If it isn't, you end up with a corkscrew flight pattern. This stability is easy to achieve, if the vehicle is symmetrical and all is aligned with the central axis. If not, you really need to make sure that the thrust is aligned with the CG. Hang the flight ready spaceplane from a string. The CG will be on the extension line of that string. If done from multiple points, you can get the exact CG from the intersections. I would guess the plane is symmetrical left and right, so the only unknown for the CG would be top to bottom, which effects the "pitch" axis. That's probably the most difficult one to get the motor aligned to, on this model.
Here's an afterthought. Just hang it from the center of the motor, if the string is perfectly aligned with the motor, you're good!
3 - In the thin air, this may not be an issue, but I have had 1/4" plywood fins completely break off an "L" powered rocket. It was due to turbulence/instability from a mirror shroud for a camera. It would be very difficult to break those by hand, but the aerodynamic forces snapped them clean off.
The wings on Vulture 2 look much more fragile. It might not survive a J motor flight at sea level, but maybe ok at 100,000'. Since there is little drag up high, your acceleration will be based on weight and motor impulse. If Vulture 2 weight is 4 pounds, final speed in thin air might be about 400mph on a "I" motor and 600mph on a "J" motor. Maybe the aerodynamic forces would be equivalent to less than 100mph at sea level. In that case you should be ok.
Wish you the best for a great flight!