You could assume that the Skyvan could be in a slight dive...i guess something around/in between 200/300Km/h?Is the speed and acceleration in relation to the aircraft known? Can it be worked out?
Some time ago i remember stumbling into an article on the Swedish MFI-17 Supporter (trainer that was also pushed into the COIN role), the Supporter could make use of the Bantam Missile, an extremely portable ATGM that was often seen being carried by infantry or maybe in some vehicles, like you state, since the Bantam was wire guided it was limited by the 2km(ish) wire that was being carried in the plane (sure it could go a bit faster, but range remained the same, or a bit less).Slant distance? If that was wire guided, the length of available wire would limit the range
As far as i know the SeaCat wasn't wire guided, could a wire guided version had been envisioned/Planed/tested for this? (or for its use on the Wasp helicopter?). Then is if the missile's Warhead was modified to avoid self destruction after a certain flight time?
Sometimes i wish we could get a few more sources about it