Secondly, I look at these mounts as a metal studded rod mount with MDF spacers, the metal rods (in tension?) provide a lot of stability, the MDF (in compression?) doesn't really have any where to expand into.
Correct, which begs the question is all that mdf needed? I would have just used the front and back plates and then spacer tubes on the threaded bar. But as John said, I think analysing MDF to this degree is unnecessary as it will typically be a first build.