Just a point on future finance and box-sets/marketing etc. The steel-box of the Superman movies has 1,2,3,4 and Returns (amongst loads of other stuff) - so I don't think that will enter into anyone's thinking as a reason not to retcon A3 and A:R.
Of the stuff I'm reading here, I still haven't seen a better retcon reason than the Nightmare. Ripley 8 now older and a robot or cloned Hicks? Not sure that works for me to be honest, it seems more tenuous than the nightmare idea. BUt that's only my view, i have to say.