(...) When exactly were both arms held by two different people?
I don't know what different people where doing exactly and when. This doesn't matter.
I would be almost certain that the gripping of her mouth stopped only by the moment she was stabbed, because it shows her attempt to scream, and I am sure this grabbing lasted some time, because the murderer evidently had very strong reasons to cover her mouth with force, strong reasons are persistent, they don't last just 20 seconds.
The problem is, too many things have to occur like a coincidence in order to have this picture includings a person trying to scream who doesn't, an attacker whith only one free hand and a weapon, an undressing of the victim followed/including a sexual manipulation, so a forced opening of more than one piece of cloths, evidence of prolonged holding of her hands and
no sign of contact of her hands with the murderer as shown by the
absence organic matter under her nails (don't miss this detail) with the perfect state of her nails, no signs of struggle/attempt to escape in the room, a stabbing on two different sides with change of position/orientation of the blade. Bear in mind that this is a sexual murder: it should be thought as the development of a botched sexual assault.
If you like, you can put in the picture the aftermath: three towels brought in the room to soak up the blood, the body moved and covered after some time, both cell phones stolen (who
knew Meredith possessed two cell phones?), an isolated bloody mat in the bathroom lying on a clean floor, a 26 centimentres blood smearing on the bathroom door (residual of a cleaning), a murderer (Rudy) with several cuts on his hands, who doesn't leave a single drop of his blood in the bathroom where he washes his hands, and no prints on the duvet he uses to cover the body ...
Obviously we are always talking of small sectors of the evidence, pieces of the whole puzzle.