I don't follow how on the basis of this you can conclude:
"MXOLAL of Isaiah 53:5 is certainly not 'pierced'."
Even from your etymological comments it is clear that the word can refer to
an act that opens up some cavity.
Many translations of Isaiah 53:5 have 'wounded' and with this the LXX agrees.
The context of 53:5 points to an act of violence or aggression (this one is
crushed, chastised and subject to blows). Another verse in close proximity
that seems to have a comparable context is Isaiah 51:9 where the word is
translated 'pierced' by many translations including JPS, ASV, RSV, NJB.
It just seems to me that your etymological comments actually support the idea
of a piercing wound at 53:5, could it not refer to 'a deep (not external)
wound that opens up a cavity' (though this may to some degree close up as the
weapon /instrument is removed, it will remain until such time it may heal)?
Whatever the case, and whatever translation one might prefer, 'pierced' seems
to me to be a legitimate translation option along with others.