Unless your spies come with some high tech precision tracking devices, you cannot track precisely enough based on human sight and guesstimation alone. An attempt to blind hit a fast low altitude jet with an unguided missile is extremely difficult EVEN if both sides agreed to COOPERATE in such an experiment. Are we going to assume that each spy. which are basically civilians in guerilla setting, have a superhuman computer like accuracy when it comes to estimation?
As the planes fly at night, you can only report lift off, but you can't report heading. Are you assuming that the planes will immeditely head to their target's bearing the moment they lift off? That normally does not happen, because mission sorties do not occur in a straight line to the target. Even if you have a special path towards the target, the plane at takeoff will not be heading to the target directly, but take a detour here, then a detour there. This special route would have to be completely mapped. Are you going to assume that there will be spies conveniently located at every juncture of the mission path to report the plane's progress? Furthemore, this is at night, this is extremely hard to spot with a naked eye if the plane has no lighted beacons and lack an afterburner.
Then there is the issue of the proximity fuze, which won't be live if the seeker isn't live. And if the seeker has to be live, the missile will be seeking and it certainly won't be traveling in a straight path. Either that or you have to set the fuse on live while in the ground, which is assuming that you will be bypassing the guidance. The commander of the SAM battery on his interview noted how he took much pains to preserve his precious SAMs, and given that, he certainly won't be launching the missiles blind because you have a very high chance of a miss if you do.
Blind hitting a plane is extremely difficult to do with one or two missiles, much less you launch an MLRS right at the plane's path, which is at least a better option. Even then that's a much better job for AA artillery since AA can saturate a plane's path, but which no SAM can. In fact, you might as well shoot rifles in the air, that has a much better chance of hitting something along a path blind than a rocket.