yeh this happened to me when macquarie bank gave me a loan approval and then before settlement they decided they weren't a bank anymore so I couldn't settle. mine was also interesting as the deposit bond company had gone belly up as well.
they can and almost certianly will go you for the difference. Am not sure about your stamp duty position but in WA for example you are often better to walk than flip because you save the stamp duty and agents fees.
the developer of course must not have made any materials changes (that is, material to you, not anybody else) to the strata plan that they deliver. the contract must be tickety boo as well. they should aim to mitigate your loss and achieve the best sale price possible and proove this was done
But wouldn't this just be attempting to use lawyers to weasel out of a binding contract on the basis of almost-certainly spurious technicalities? (I mean, if everybody went around doing this, not much building business would likely proceed in future, would it?)