I agree it getting increasingly hard to suspend one's disbelief about this.
That said, Jigsaw himself has always been a manipulator of others and has often had help. We now know Hoffman was recruited early and that Amanda was helping even in Saw I. We know in Saw I that Zep was involved at least in the abduction of Dr Gordon's wife. The final trap of IV was being run by the lawyer, Art, at least in part. In Saw II, some of the kidnappings were performed by Obi, who ended up in the nerve-gas house anyway. Obi also appears in a flashback in Saw III. He may actually be more like Amanda than like Zep; a failed apprentice rather than purely a dupe. There may be other apprentices we don't know about, even people who survived being tested and didn't publicise it.
I think events have always involved a lot of chance. For example, in Saw I I am sure Jigsaw did not expect the detective Tapp to break in on Zep at the Doctor's house, then chase him to Jigsaw's lair only to be killed at the last moment.
I am coming to believe he was being pragmatic, and taking out people who were liable to stop him. Jigsaw would not allow himself to be taken. (Notice when he tells Hoffman he can turn him in, he has the double-barrelled shotgun with one barrel loaded. If Hoffman had decided differently, Jigsaw would have killed him.)