It's one of the fundamentals of the Swedish legal system, you can't be sentenced in your absence or at least not without being heard

He was heard at the embassy, back in November.

Not by Swedish police or prosecutor, Ecuador didn't allow it. Only a prosecutor from Ecuador was allowed to ask questions.

That does not limit the amount of evidence that can be collected.

It does not but it limits the possibility to prosecute which is what the prosecutor came to terms with, at least according to her comments on the Swedish evening news. Basically the statue of limitations for this crime is 10 years, it's now been 7 and there is no resolution in sight so it's basically just a waste of time and money. Embarrassing if you ask me but that's the way it is.