Is there no "standard" string to play the mute?   I thought the video using the "D" might reflect Scandinavian experience ?

The underlying message is that a violin CAN be heard with a tourte mute AND, SOMETIMES, the violin sounds better.

She is happier and has demonstrated the "good ear".  Most student violins give too much treble which is mainly what a tourte mute reduces.

This taming of the sound also encourages better intonation.

Then too, the under-the-chin sound is more pleasant for the player.

Not bad for a little piece of rubber

Tell ya wat Mad.  Because you are an esteemed member of this forum I will let you see a classified photo.  The photo shows tourte mutes cut to various shapes to achieve different pleasing tone effects particularly with an instrument of the sub VSO variety but not excluded from use with better instruments.

Experiment at will but don't cut your fingers.  You can secure any little rubber thingey to the bridge with Elmers Paper Glue. (removable).

Keep all this secret.

I do use some and I can move the sound of a violin around very much as if I reset the sound post.

I would suggest top left version on either the "A" or "E".

A razor blade is good to cut the rubber but don't cut your fingers !