The fact is that after 15 September 2007, the PJ would not have been able to constiute the McCanns or Murat as Argidos, becuase they did not have any proof of involvement. Of course if they wanted to ask them certain questions (like the 48 questions) then they would not have been able to without the McCanns having arguido right, and for this they would have needed some proof beyond suspicion.
I don't think they could lift Mr Murat's arguido status but, yes, they wouldn't have given the McCanns the benefice of that status.
Now, no, the PJ had the right to interview them separately as witnesses and ask questions like "can you describe exactly what you did when you entered in the flat on the 3rd around 10 ?". Mrs McCann, of course could have lied, this is a right !, but she would have had to answer.
Ah, btw, their request to be assistants in the process, which had been accepted, would have remained (the MP suppressed it, judging it was incompatible with the arguido status, in fact it would have mainly be redundant).