I cannot really speak to your question about QM's pearls, but your post intrigued me greatly.
I do not ever remember seeing this "Queen Alexandra-ish" coronation portrait of Queen Mary - with too many jewels to count along with all those ropes of pearls.
So I did a quick image search and was astounded as what is being posted as "coronation portraits"...there are so many variations.
Even if you pare out the Durbar posts - there is a lot of variety.
My guess is they did some extra sittings - but not exact replications.
My question is -
Do we know which one is actually correct? (i.e. worn in the Abbey)