Hi, Jim:
I will reliterate my question on what you have found.
(i) First, it doesn't make sense that we have so-called ablative
case marker /-to/ beside the regugular ablative case endings.
(ii) Historically, the so-called ablative /-to/ must have developed
from a regular word, which has then transformed to be a so-called
  postpostion /-to/, which assumed the ablative sense as its primary
  function.
(iii) The question here is why did the Pali speakers want to
  have the /-to/ postpotion besides the regualr ablative endings.
(iv) My guess is that they needed at least one ending/postposition which
  is flexible in its (case) use (due to a metrical reason?).
(iv) In other words, the so-called ablative /-to/ is not ablative
at all, but should be regarded as a kind of "universal"
case marker.
(v) If the above hypothesis is correct, then, we should be able to
predict the following two findings:
     (a) regualr ablative case endings are used always as their
        ablstive senses
        (so that you would never say/find something
         like: saara.m saarsmaa ~natvaa; meanwhile
     (b) we would be able to find numerous examples of the use of the /-to/
        as an uninversal case marker (functioning as nominative down to
        genetive).
(vi) If both (a) and (b) are supported emperically, then, we can
  conclude that the affix /-to/ should neither be regarded as a mere
  ablative marker, nor be as a case marker (in the sense that
  it is a universal postpositon, foreseeing the future of the end of
  the inflectional languages in India, as being manifested in Hindi
  or Punjab).

  Does my arugment make sense to you?  tadao