But where does *-mi come from?

Some nominal and verbal suffixes are identical, as Alscher has noted,
e.g.

locative *-i and primary *-i
dative *-ei and middle *-oi
'locative' *-r and middle *-r
[endingless locative and secondary?]

Latin has a "double ablative", a dependent construction where a
subject and a past pasticiple (*-tó-) of its verb are given ablative
endings. Similar constructions exist in OCS (with dative) and Sanskrit
(several cases). Perhaps such a dependent construction is the origin
of the mi-conjugation?

Torsten