That is because Israel was not conducting artillery strikes against origin sites of those rockets due to international pressure. SoP in case of a rocket/artillery strike is to saturate source with counter battery fire, which if done in Gaza would mean death of every Palestinian and international sanction and condemnation that accompanies it.
All saturation fantasies assume that whole of IAF and rest of SAMs (Spyder, Pantsir, Akash, S-300, whole zoo of soviet era SAMs) and Anti-Aircrafts guns would take a day off so that PAF could have a fair fight against S-400.
A dud and working missile cost same. You could field different systems ,one costlier and one cheaper, but in that case Radar would be able to make out which is what and leave interception of 'dudder' system to cheaper interceptors or AA guns.
MTCR is a missile control regime, not even a treaty. There is no legal obligation on anyone to abide by its rule so no waiver would be required.
Also SAMs are not in export control list of MTCR as they do not satisfy criterion on MTCR.
https://www.armscontrol.org/factsheets/mtcr
Or you could take a look at whole annex:
https://fas.org/nuke/control/mtcr/text/mtcr_handbook_guide-annex.pdf
Some parts of S-400 are listed under annex-II ,which urges members to consider end use, and this is implicitely not applied on SAMs.
I think that this talk about drones need to be settled.
CH-3/Burraq has a speed between range of 120-150 Km/hr, with a maximum speed of 220 Km/hr (claimed, never observed). IAF has three AA guns in service: ZSU-23-4 and ZSU-23-2 having a muzzle velocity of 980m/s, and Bofors 40mm with muzzle velocity of 1030 m/s.
Assume that Burraq is flying at its maximum possible altitude of 4Km: Applying newton's law, following is the time AA gun shell would need to reach Burraq, and the distance Burraq would have moved, even if it is traveling at its maximum possible speed, and the degree of deflection needed by the shell:
ZSU-23-4/2 :Time= 0.01457 second, Distance moved by Burraq at 220 Km/hr = 89 cm, degree of deflection: 0.8014 seconds.
Bofors 40mm: Time= 0.0138 second, Distance moved by Burraq at 220 Km/hr = 85 cm, degree of deflection: 0.763 seconds.
As you can see, even at its best, a CH-3 is as good as a stationary target.
Moral of the story: You pay peanuts, you get monkeys. If there would have been cheap alternatives to SAMs, they would not have come into existence. All countermeasures against S-400 are very costly and at this time onle USA (probably Israel?) possess them.
Terrorist do not even have AA guns. They could even be bombed by this: