Hi @lapetite ,
One property, one connection sounds like an all too common NBN restriction (affects more than just Optus customers though). FWIW people with FTTP can have up to three seperate providers at 100Mbps each to a property as it was designed that way from the outset. I was recently talking to a person on NBN satelite plan (max 25Mbps) in exactly the same situation. With only that much bandwidth they couldn't even share between dwellings.
Optus shutting their network down three months after NBN arrives has always been the case. They made this arrangement when they sold the HFC network to the government. IMO its not wholy unreasonable, as if they wait 18 months they may end up running a massive network at huge cost with only a few stragglers remaining online. They can't make an exception for just the od houses.
The NBN does have its faults but one of the big benefits is flexability. Before only Telstra & Optus, now every RSP can offer options. In your case you should be able to find a very workable solution. I'd suggest:
1) The wifi router as you suggest. Something like Ubiquiti Nano is cheap and powerful.
2) Better and more reliable (and faster) could be just running a 30m ethernet cable between the dwellings. Plug one end into the NBN router and the other at your place. You can just bury the cable a few inches with no fuss. Add a $25 5 point switch at your end and plug in something to do wifi. Get a 100Mbps NBN plan to share (50% saving).
3) Depending on where you are you could also grab a 4G wireless broadband plan. Doesn't suit all but a definte option and no wires.