As for Kurono being the guy in the Gantz ball, its possible if Gantz is from the future, as the guy in the ball looks to be in his thirties-ish. Obviously the game is not canon, but its a fun fact.

Just cos Suzuki didnt know of Gantz doesnt mean the Japanese government didnt have agents in its ranks that did know. Plus, correct me if Im wrong but the Oni were the first aliens (with exception of the final Chibi out of hunt circumstances) that revealed themselves to the public in their true forms (The Tanaka had gone to shops to get candy capsules in their robo-suits making people think they were high-end cosplayers) which may have have led to Gantz to blow the cover. Plus their cover was not revealed until only the Oni boss was left.
As for insiders, the original Tokyo team might have been agents.