Historical Explanation for *pi as Plural and Proximate and Nominalizer

[are2 at buffalo.edu]

John,
Hey!  I'm not sure I follow the problem of pi being plural.  It is a
pluralizer in Lakota as well (as Violet Catches (Thanks Violet & I
hope I am not misunderstanding/misrepresenting your information)
pointed out in her explanation, it's used with 'they').  Sometimes,
I'm dense with the historical though.
I think that analyzing pi as plural actually leads to a very coherent
grammaticalization pathway to its role as proximate marker.  I pasted
a piece of the diss below which discusses it.  It's not a final
version (God grant that someday such a thing will exist); comments are
great.
Best,
Ardis

...
Third person plural verbal marking is also used as a backgrounding
device.  That is, when the subject is unimportant/unspecified, third
person plural verbal morphology without an overt subject NP is used
(51).

51. Khi     Itigonthai   akha    monzhon     thon    wethinwin-bi  a-
i....
   And Grandfather   the     land           the          sold it- pl
he said-PL
   'And Grandfather said that the land was sold ...'	(Dorsey 678.1)

In the subordinate clause in (51), the land is of central concern and
the person(s) selling it are backgrounded (also reflected in the use
of the passive in the translation).  In the Omaha construction, the
third person plural subject governs the plural affix and the third
person singular object ‘land’ is zero-marked.  The plural subject,
which has no overt NP and is relatively unimportant, co-occurs with a
singular object which has an overt NP and is of central concern.  Such
occurrences could be re-analyzed as a singular object governing a
plural morpheme.  Another example of a singular NP occurring with
plural verb marking (which refers to a subject without an overt NP) is
given in (52).

52. Egithe         itonge thinkhe tizhebegthon gaxa-bi-ton-ama,
      It happened sister  the            door       make-pl-AUX-EVID

      a       khe agthonkonhon konton-bi egon     ubatihetha-bi-ton-
ama.
Arm the   on each side       tie-pl having hung up -pl-AUX-EVID
‘And behold their sister had been made into a door: having been tied
by her arms on both sides, she had been hung up.’ (JOD 81.19)

In (52), again a singular object co-occurs with plural verb
morphology.  (And again, passive voice is used for translation.)  Were
the plural to be re-analyzed as marking the object ‘girl’ in some way,
it could not be marking number, but rather must be marking some sort
of discourse status (what is of central concern).  A pattern of
marking third singular subjects with the 'plural' to show discourse
status (rather than number) could logically result from such a
reanalysis.  As all other person forms overtly mark the verb, this
reanalysis is possible only with third person singulars.
...