As far as MS goes remember that their Myelin is made form oligodendrocytes so it is not the neuron regenerating, indeed if the neuron gets damaged that the oligodendrocytes tracks along it won't regenerate.
Also even the regeneration in MS isn't always perfect, same with guillian barre syndrome, the myelin comes back but it's never quite the same. Also, I'm not a 100% on this someone correct me if I'm wrong, but with MS it is part of the myelin they is destroyed not the whole thing, if you loose up to a certain percentage of it on a neuron then function ceases and you can loose the neuron if I remember right. Basically like MS causes inflammatory damage to sections of myelin that alter function but those cells can repair the damage if it wasn't to extensive.
As for the other ones I think there is some confusion from the regenerate part. So permanent cells are stuck in G0 like you said, thus they can not divide themselves, i.e they will not produce daughter cells. Now some, like skeletal muscle, have stem cell populations that can regenerate to some degree (satellite cells for skeletal muscle) but the permanent cells that are lost. Think after MI what happens to the myocyctes that die from ischemia? They are replaced by scar eventually. The stem cells though are their own cell line, a myocycte or a neuron cannot go back into the cell cycle and regenerate it is that separate stem cell line that would do that.
Hopefully that makes sense. So permanent cells are one cell line that is stuck in G0 and cannot go back, stem cells are another cell line and can make permanent cells but once they make that switch, they can't go back.
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