Earlier quoted context omitted.
Your original comment made it sound like they had no input in the questions being put to Assange. Merely "present" to observe Ecuador. This article suggests Sweden was involved and I don't see anything suggesting the procedure (under "an agreement the two countries made") was problematic. Who asks the questions seems to just be a technicality here.
"Isgren is allowed to ask Assange to clarify his answers, but not to put additional questions, and will receive a written transcript of the exchanges from Ecuador after the interview has concluded." - https://www.theguardian.com/media/2016/nov/14/julian-assange... This is problematic.
But this raise a primary question. Is the distinction of being able to instantly make follow up questions of such importance that it will swing the binary choice of charging Assange of a crime or dropping the case?
The primary cause for a guilty and not guilty verdict should not be the quality of the environment for a police interview. Not for a such serious allegation like this.