Re: wild rotation?
but wouldn't that require the 'fixed' portion of the gun to have sufficient mass to balance the reaction effect. If not it is as likely to propel the tube, girder and balloon assembly backwards as it is to push the plane forwards. And as previously stated that could swing the girder and alter the angle of the launch.
Better not to have the back plate and rely on the reaction motor entirely...
.. or go back to the vertical launch paradigm. (three balloons, vertical launch tube through the middle, swing wing glider at the bottom of the tube, discarding sabot to reuse the gas pressure in the launch tube (nice)).