Somehow I can't think of Gantz as being evil intentionally.
So, maybe there is a reason the targets don't show all at once.
Like, maybe for the group to get a feel on the enemy's weapons and weaknesses. Yes someone gets blown off in the process, and Gantz doesn't care, his "logic" is that the real persons are already dead so no harm done.

No instruction manual, maybe because no one ever asked for it?
Possibly if one was to ask, Gantz would provide it. There have been times in the manga when Kurono haggled Gantz for something and got it in the end.

The trap/capture gun not used, how is that Gantz's fault? If I remember well in the Buddha story one guy tried the capture gun on a Buddha but the Buddha avoided it. The capture gun is likely to be used at the end only, on a weakened enemy, not as an all-around weapon, I think. It's a slow weapon.