I think the republic was over long before that, people just didn't realise it. (Sulla, Pompey etc had all put their nails in the coffin).
With regard to Octavian, It's something I often wonder about - he was a great Politician and manipulator, who's to say what he would have done without being Caesars heir. I agree, it would have been extreemly difficult for him to take and hold any sort of supreme power, guess we'll never know.
On another note, I think the fact that Octavians rule went for 40 years and garunteed that when he died, next to no one was alive to remember living in a republic went a long way to ensuring that the concept never rose again in Roman politics. I like to think that Sulla, Pompey, the Oligarchs, and Caesar all put nails in the republics coffin, and then Octavian/Augustus, buried it.