It should not matter that the riser does not border the back wall all long as it is enclosed. Otherwise it cannot be a Helmholtz resonator. It also should not make much difference which side the ports face. Remember that the effectiveness of the resonator depends on the area of the port so don't make it too small.
You can do CLD on existing walls without going to the studs, t just won't be as effective.
Todd. In my software the absorption is input as Alpha, the absorption coefficient. This would almost certainly never exceed about .5 at LFs and even that would be very difficult. That said, with floating CLD walls I suspect that a value of .5 is achievable. In my room strong modes are hardly evident al all except to the lowest one at about 25 Hz. Above that no strong modes are apparent at all. This is not typical of course, but it is doable and I prefer to work with the better environments, not the typical ones. The low damping simplification is probably violated at values much above about .05-.1, almost no damping at all.