Here Joseph Smith, in his editing for the 1837 edition, changed the difficult where unto to but. He probably chose but because it expresses the implied contrastiveness between the two clauses (with their parallelism offset by the not). The 1830 compositor set the expression where unto as the single word whereunto, although this may not have been necessary. There is no other instance of where unto (or whereunto) in the text. Here where unto seems to mean something like ‘with respect to which’. There doesn’t seem to be much evidence in the Oxford English Dictionary for such a specific use of the word whereunto, although under the related whereto the OED lists definition 3b, ‘in addition to or besides which’, which suggests a type of conjunctiveness that might work here in Ether 12:23. Despite its difficulty, the earlier where unto does appear to be fully intended and will be restored in the critical text.
The 1830 spelling whereunto should probably be avoided because its specific uses in the King James Bible do not work here in Ether 12:23. In the King James text, whereunto can be used as an interrogative or as a relative pronoun, with meanings like ‘to where’, ‘to what’, and ‘to which’, as in these examples:
Perhaps the two-word spelling where unto will lead the reader to a more literal interpretation here in Ether 12:23, something like ‘with respect to which’.
Summary: Restore in Ether 12:23 the original use of where unto, despite its difficulty; here it seems to mean ‘with respect to which’.