Let's take those arguments one at a time.(1)  Assange has not been charged with any offence.  Correct - if by charge you mean formally indicted.  In the Swedish system formal indictment takes place at a very late stage in proceedings, following a second and final interview with the suspect, and in the case of a person in pre-trial detention, trial must follow within two weeks.  (As Assange is very unlikely to get bail again, for obvious reasons, it must be assumed that this would apply in his case).

However, the High Court has held that Assange does stand accused of the four offences (including rape) for which his extradition is sought.   It is a requirement of the Extradition Act 2003 that the warrant contains a statement that the person in respect of whom extradition is sought stand "accused" of the offence(s) set out in the warrant.  There is no doubt that the European Arrest Warrant issued in Sweden did contain such a statement.  

Assange sought to argue, first in the Magistrates Court, then in the High Court, that it is not enough that the statement is made, but the statement must also be true; and that he has not in fact been accused of any offence in Sweden because he had not been formally charged and so criminal proceedings had not yet commenced.  He lost that argument.  For full details, see the judgment in the High Court, especially paragraphs 128-154 which deal with Issue 3, Was Mr Assange accused of an offence in Sweden?

At para.154 the High Court concludes:"Although it is clear a decision has not been taken to charge him, that is because, under Swedish procedure, that decision is taken at a late stage with the trial following quickly thereafter. In England and Wales, a decision to charge is taken at a very early stage; there can be no doubt that if what Mr Assange had done had been done in England and Wales, he would have been charged and thus criminal proceedings would have been commenced. If the commencement of criminal proceedings were to be viewed as dependent on whether a person had been charged, it would be to look at Swedish procedure through the narrowest of common law eyes. Looking at it through cosmopolitan eyes on this basis, criminal proceedings have commenced against Mr Assange."  [My emphasis] Therefore, Assange lost on this issue.  There was no appeal on this issue to the Supreme Court.  

(2)  Assange is ONLY wanted for questioning at this stageThis is not true and was not even argued on Assange's behalf in the High Court.   

It is a requirement of s.2 Extradition Act that "the warrant is issued with a view to his arrest and extradition ... for the purpose of being prosecuted for the offence."  As the High Court notes at para.129 of its judgment, "It was common ground that extradition is not permitted for investigation or gathering evidence or questioning to see if the requested person should be prosecuted."  So if the UK courts accepted that Assange was only wanted for questioning, they would not have ordered his extradition.Assange did argue in the Magistrates Court that the warrant had been issued for the purpose of questioning rather than prosecuting him.  In response to that argument, the Prosecutor, Marianne Ny, provided a statement dated 11 February 2011 explaining the Swedish procedure.  (The High Court quotes from this statement at para.142 of its judgment.)  She concluded as follows:

"Subject to any matters said by him [in the second interview] which undermine my present view that he should be indicted, an indictment will be lodged with the court thereafter. It can therefore be seen that Assange is sought for the purpose of conducting criminal proceedings and that he is not sought merely to assist with our enquiries."

The Senior District Judge found against Assange on this point and Assange did not pursue it in the High Court.  At para.131 of the High Court judgment: "It was accepted in oral submissions made on behalf of Mr Assange that the surrender of Mr Assange was sought for the purposes of conducting a criminal prosecution (satisfying 2(3)(b)), as the Senior District Judge had held.  That concession was made because it was accepted that the words 'for the purposes of being prosecuted' were broad enough to encompass a prosecution that would commence in the future."  [Emphasis added]   Rather, it was argued that as an additional safeguard the proceedings must already have commenced.  As already indicated, Assange lost on that point.

(3)  Sweden recently interviewed a murder suspect in Serbia; why can't Assange be interviewed in London?This argument has been doing the rounds on Twitter recently.  Here's the newspaper report they commonly link to.  (I am linking to the Swedish original but obviously you can use Google translate if you want a rough idea of what it says in English).

This report dates to 22 March 2012 and says that Swedish investigators have travelled to Serbia to interview a 21 year old man suspected of the murder of a 26 year old man in Uppsala.  However, there is nothing in the report to suggest that this is the second interview prior to charge which Assange is now required to undergo.  The report says that Swedish police and investigators "have now interviewed the 21 year old man" which clearly suggests they had not interviewed him previously. 

In Assange's case, he already had his initial interview on 30 August 2010 before he left Sweden.  So to compare his case with that of the 21 year old interviewed in Serbia is simply not comparing like with like.  

A later press report indicates that the murder suspect was extradited to Sweden on 26 June 2012:

A third report in July 2012 makes it clear that the man had still not been charged with any offence at the time he was extradited: 

This report indicates that the 21 year old suspect has been further interviewed since his return from Serbia and that any formal charge must be brought by 26 July 2012.  Therefore it is clear that at the time he was extradited on 26 June the suspect had still not been charged nor had he undergone the second interview.  Plainly the interview that took place in March 2012 was an initial interview and not the second pre-charge interview that Assange is now wanted for.  The second interview did not take place until after extradition.

ConclusionAssange has not been formally charged with any offence but he does stand accused of the four offences, including rape, outlined in the European Arrest Warrant and criminal proceedings are already underway.  The European Arrest Warrant has been held to be valid by the highest court in the land.   Assange and his supporters are now raising arguments which he either lost or conceded in the lengthy UK proceedings on the application for extradition.  

It is not true that Assange is only wanted for questioning.  The next step in the Swedish proceedings is to conduct a second interview with him before making a decision whether to formally charge him.  The prosecutor is presently disposed to charge him, unless any new evidence emerges that might change her mind.  If a decision is taken to formally charge him, Assange would face trial within two weeks of that decision being made.  It is difficult to see how this could happen if the final interview takes place in the Ecuadorian embassy in Knightsbridge.  Even if he were interviewed in the embassy, if a decision was then taken to formally charge him, it is somewhat difficult to believe that Assange would suddenly renounce his claim to asylum in Ecuador.  

In these circumstances it is difficult to see why Sweden would or should agree to interview Assange in London rather than continue to push for extradition so that they can follow their usual procedures in due course.  No other fugitive from justice gets to bargain with the authorities about the way in which their case will be dealt with.  I don't see why Assange should be any different.