I think if Mercedes want to object, article 48.12 of the sporting regs seems to have been half followed:
48.12
...
Having overtaken the cars on the lead lap and the safety car these cars should then proceed
around the track at an appropriate speed, without overtaking, and make every effort to take up position at the back of the line of cars behind the safety car. Whilst they are overtaking, and in order to ensure this may be carried out safely, the cars on the lead lap must always stay on the racing line unless deviating from it is unavoidable. Unless the clerk of the course considers the presence of the safety car is still necessary, once the last lapped car has passed the leader the safety car will return to the pits at the end of the following lap.
So, it seems that only once the last lapped car has passed, the safety car should've pitted at the end of the next lap, which would've been lap 58. At least that's what the rules appear to say to me.