I still don't think its either a,b or c.
I was always told that a horse that has foam is a good thing. Even when I saw the Lipizzaner horses, the announcer specifically said that the foam was a good thing.
Both my mare and gelding will play/chomp on my bit. Its a myler bit. I know it is fitted correctly, and you yourself suggested myler so I don't think its the bit. And there was no contact.
This website seems to think that foam is a great thing to have.... Horse Foam at the Mouth