That is the most adorable picture I've seen in a long time!
You mentioned that if you walk away to grab the camera, the mouthing stops. You have literally answered your own dilemma in that one sentence!
I realize that drop it, or "let go" is sometimes more of a struggle than leave it. Believe it or not, my wee Oliver (one white eye) is absolutely relentless when he has something in his mouth that he shouldn't, or that we wish him to "let go"! Of all the dogs I've had, he is the hardest one to break when it comes to this. Henry, on the other hand, literally spits it out...good boy! Oliver clamps down with a death grip that is hitherto unknown for me.
With my larger breed dogs, I could easily pry open their mouths and take the offending object out, along with the command and a "time out" from any attention or whatever. With Oliver's wee mouth, it is nearly impossible as there is not enough "muzzle" on which to hold and insert my fingers at the back of this mouth, thereby forcing his mouth open. Not only that, but he has also been known to give a wee nip like response in defiance. He is much smarter than he looks.
Rest assured he has never bitten anyone, but his jaws clamp shut like a steel trap. I think Boucane is far less likely to be so resistant and being a baby, he will learn what it is you expect, much like Henry has.
We actually caught Oliver trying to pull a placement from the table with food on it...this past week! If you know Oliver's history, that is rather surprising as he has major health issues. While being admonished for trying to steal a plate of food, he gave a growly nip like response to my older son who is nearly 20.
The admonishment was entirely verbal along with simply moving his body gently from it's location. "Bad dog, get down" type of thing.
Boucane is definitely not Oliver-esque and will learn to use his mouth gently, for loving purposes only. With everything I said about Oliver, he has never nipped anyone and would never because he knows I won't have it. Boucane is more like Henry and will do whatever you ask, eventually, simply to please. Be consistent and allow room for some mouthing as that is a form of loving, whether it's licking or "holding your hand".
I am looking forward to seeing some videos, but I think you are on the right track. I only use Oliver as an example as most people wouldn't believe a sweet, little Cavalier could behave that way....including myself!