Hm, perhaps, but what about other options? What about interviewing him in embassy in person or via Skype? Is there a reason that would be impossible for Swedish government?
I've not heard any compelling explanations why this hasn't been attempted, though IMO, Assange would be a fool to agree to it -- guilty or innocent.
On the other hand, there's little doubt that the attempted extradition procedure has correctly followed current UK law: it went all the way to the Supreme Court, and he was represented by absolutely top-class lawyers. So Sweden has the right to do what they're doing.
People have been extradited from the UK on 'European Arrest Warrants' for crimes such as drink-driving and theft of a piglet, with no prima facie evidence required.