Why do you need to see the body?
Don't be ridiculous.
We can watch that casket all day, but we're not guarding the body unless we know the body is in the casket.
The shell game is the oldest con in the galaxy. The mark watches the shells, but the bean under the shells gets swapped out.
In order to steal the body, the first trick is to convince everyone that it's still in the casket.
At least that's how I'd do it.
I thought your plan was the smash-and-grab.