An Australian Labor government is not going to buy nuclear powered subs. They would only consider that down the line if AIP proves to be impractical (and from what I've read here there is a lot of confidence in AIP).
From what I understand, Australia hasn't shown any serious interest in moving towards AIP. I'm paraphrasing GF here but if I recall correctly a small number of AIP systems were acquired with the intention of assessing potential use on the Collins class, but following the decision not to fit the technology to any boats in the fleet, these systems were subsequently stored and did little else besides gather dust. For whatever reason, AIP was not seen as an imperative. It's blind guesswork on my part but I'd speculate that the benefits of such a system would be substantially diminished once Collins had demonstrated the capacity to meet range and capability requirements without it - particularly considering the submarine had other, more fundamental issues that demanded massive effort and expense to fix. Or maybe it was other factors, as I said I'm just guessing so I'm happy to be corrected. Hopefully GF will jump in here if I've got the wrong end of the stick.
Regarding the Collins replacement, given the projected increase in tonnage from Collins and potential for advances in fuel cell technology, it seems to me that AIP as we know it will likely prove unnecessary. It all remains to be seen, of course... technologically a lot is going to happen between now and then, to the point where I don't really think I know enough to speculate on final requirements, let alone the resulting capability.
I agree with you on the Astutes, no chance. The Royal Navy would have to be crazy to let any of them go, and besides that I don't think it's realistic to think Australia has the capacity to support two such vessels under the prevailing political climate. I hope the country can change to the extent that developing an infrastructure to support nuclear applications is a feasible option, but I don't think it's going to happen any time soon. With any luck, emerging technologies will help close some of the capability gap between nuclear and conventional submarines in the years leading up to the Collins replacement.