was augustus senile?

Tell one the truth, history seems to paint two versions of the person: the ruthless, power-climbing Octavian, and the noble, virtue-espousing Augustus (except when his family misbehave). It's just popular culture prefers to cover the road-to-power Octavian instead of the noble Emperor. I remember watching CLEOPATRA, and hoping Octavian got his due for his villainy. Then I read about him, and learn he becomes this noble great man!

Perhaps gaining power ennobled him instead of corrupted him? Still, there are traces of the gangster in the I,C Augustus: his ability to hide his distaste with a friendly facade (his scenes with Agrippa and Tiberius "Don't Sulk!"), the look he gives Drusus when he mentions that his father "Was my enemy too." His last words to Julia's lovers (from his mood, punishment ain't gonna be pretty), and sticking a dagger on Postumus' neck.

I don't see this interpretation as naive and gullible. Does he really do what Livia tells him what to do? Tell one the truth, except for the situation with Postumus (keeping him away from Livilla, withholding his inheritance, replacing him with Germanicus in the war), Augustus pretty much held his own against Livia. Despite her protests, he refused to cancel the Julia-Agrippa marriage, would not have married Tiberius to Julia without his daughter's pleas, he banished Tiberius, and kept his foot down about that exile for almost a decade, allowed Claudius to be at the Games, etc. And Livia covered her tracks in the Julia & Postumus exiles quite enough that what happened to them was pretty much Augustus' show.

Is it really naivete for him not to realize Livia's treachery when all his heirs dropped off one by one? Historically, most of those deaths actually happened (with the exception of grandson Gaius, who actually died after Julia's exile). It does seem ridiculous for all those tragedies to happen, but its probably true none of them were Livia's doing or any other treachery. If the real Augustus did not consider Livia as a suspect in what happened to Marcellus, Arippa & his sons, then shouldn't I,C Augustus be given the same respect for not suspecting?

The end of "Waiting in the Wings" has Augustus begin to lose his wits after the multiple tragedies.

As for Postumus, the novel has Augustus pulling a switcheroo and got his grandson out of the island. Later, Postumus managed to meet up with Claudius, but Livia's finds out and has him captured and tortured to death.

the real augustus was a hard, ruthless man. i claudius portarys him as a lovable old buffer, but he was nothing of the kind.

My memory might fail, but in the book Graves wrote that they took Postumus off the island and left a guard that was very much like him, so it was the guard that was killed, but then when Postumus tried to return to Rome he was killed anyway, he only had time to send Claudius a letter.