> Israel is an occupier.

Not really relavent. Occupying powers still have the right to self-defense. Certainly they have the right to take defensive measures to prevent attacks on the civilian population of their primary territory, which is what is being discussed here.

> Israel won't let food into Gaza in reasonable quantities

As far as i understand the food situation in Gaza has now stabilized. However even if Israel was illegally restricting food into gaza, that wouldn't have any bearing on the legality of them setting up air defense systems on their own territory.

> All in violation of international law

Being an occupying power is not in and of itself a violation of international law. (The food thing might be. Israel is allowed to put certain restrictions on aid, but groups like the ICC have argued that the restrictions were beyond what was permissible under international law. Personally, even though it is incredibly unlikeky to happen, i hope the issue goes to trial at the ICC so we get a firm answer. However even if true, it does not mean Israel loses every right it has under international law)