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...
Problematic for whom? You seem more concerned about this than the Swedish prosecutors. It seems this was the procedure they agreed to. Whether it's normal or not I don't know. But they haven't made a fuss about it. And I think they would have if it was a big concern.
They could ask for a second interview and present follow up questions.
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.
