But then, why was he not extradited from the UK when they had him in custody? That totally makes no sense: Sweden would have no interest at all in such a scheme

But then, why was he not extradited from the UK when they had him in custody? That totally makes no sense: Sweden would have no interest at all in such a scheme - they'd go against UK and the EU and their own population while the UK would loose the same in both scenarios. That leaves extrajudicial extradition, but if the US really wanted that, they'd just let Assange fly to Ecuador, nap him there and avoid illegal actions on EU soil.