I would not describe the exemptions you mention as an alternative level of membership, Zacs. They are "grandfather rights" which were secured by existing members when new proposals were made. And they were secured when the governance of the EU was somewhat different to how it is now. It is unlikely that any such opt-outs will ever be granted again. I would not say that Ireland, Denmark or the UK enjoy (or suffer) a different level of membership to the other EU members. They simply secured absolution from arguably the two gravest errors of policy that the EU has ever made.
What the UK is seeking now is non-membership but with members' rights and privileges. I know you will mention EFTA and Iceland, Liechtenstein, Norway, and Switzerland. In my view those four nations are foolish and might as well either leave EFTA and/or join the EU lock, stock and barrel. But even that is not what the UK is seeking. It wants privileges available to EU members when it leaves and that isn't going to happen.