Latin mecum, tecum, etc.

[petegray]

> I suppose -cum is a remnant of an older time when prepositions were either
> postpositions or relatively mobile adverbs.

Yes, so it is suggested.

> Is there any obvious reason why -cum is the only instance of the archaic
> system?

It only occurs with personal pronouns, and nowhere else.   The most
interesting suggestion I have heard is that it began with an avoidance of
the sequence cunn-.   cum nobis could sound like forms of cunnus, from which
comes the good English word cunnilingus.  Hence nobiscum, thence to other
personal pronouns.

> May *kom (cum) and *ko (cis) and *kwe (-que) be related etymologically around
> the idea of "here-nearby-with"?

No.  Latin cum has a good PIE history in *kom or *km, and -que has a
different one in *-kwe.  (different consonant, different vowel)

Peter