As indicated, even if it were true that Swedish government was an unable to offer Assange a so-called "iron-clad guarantee" against extradition, there is still grounds to negotiate in order to have him travel to Sweden to face these allegations; given that the Swedish government clearly has, at the very least, a significant role to play in the process, its advanced position against Assange's extradition to the U.S. on the basis of WikiLeaks' journalistic disclosures would be significant. But there is at least a strong argument to make, if not an irrefutable one, that the Swedish government is able to offer precisely the guarantee that both Assange and Ecuadorean authorities have sought in order to enable him immediately to travel to Sweden to face the sex assault allegations against him. Independently, the British government is also clearly in a position to contribute to those assurances, given the need for its consent if extradition to the U.S. from Sweden is to take place.
If one wants to find a culprit for why these sex assault allegations are not being resolved the way they should be, the refusal of these two governments even to negotiate to secure Assange's clear rights against unjust extradition is the place to begin.
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UPDATE II: For even more compelling evidence that the Swedish government is the final decision-maker in extradition matters and does indeed have the power to guarantee Assange that he would not be extradited to the U.S. based on his journalism, see the citations in Point 3 of this excellent reply to Green.