They were used by the PJ and paid, like the PJ, by the State. The PJ confirmed that their invoices were correct.
The Metropolitan police in the UK follow exactly the same method. Are their interpreters 'in cahoots' with the police?
You seem to be moving into the realms of fantasy in your attempts to attack the Portuguese Judiciary.
The impartiality question is concerned with the interpreter knowing or having a connection with witnesses or their families.
That is a bit harsh ("moving into the realms of fantasy"). I am not trying to go that far but I have worked in an environment where conflict of interest (COI) is taken seriously, so I know what it should deliver and it won't only be "the impartiality question is concerned with the interpreter knowing or having a connection with witnesses or their families". Even though that would be important as well.
If we could get to see the COI declaration we would see the types of concerns there are. How could we get one of those? Even one that is used in the UK would give us a clue.
They would not get paid directly from the PJ investigation. (I'd be surprised if they do.)
The policy is to prevent any claims of "cahoots" from either side.