So the vast majority of points lies inside. But few of them loe outside. I guess they deviate so little that this would not matter. Maybe one can move them just a little inside and maybe? For simplicity, I just move all the points of the discrete region a little bit towards the origin:
Mscaled = TransformedRegion[M, {x, y, z} |-> 0.99999999 {x, y, z}];
Count[cf[MeshCoordinates[Mscaled]], 0]
Volume[Mscaled]
0
4.40038
So now all of the discrete regions vertices lies within the convex body, but the volume enclosed by the mesh is still stays greater than 22/5
?22/5.