Latin mecum, tecum, etc.

[Lionel Bonnetier]

It's funny that Latin suffixed -cum in mecum, tecum, etc.
has created some special -go case marker in Spanish with
conmigo, contigo, consigo. I suppose -cum is a remnant of
an older time when prepositions were either postpositions
or relatively mobile adverbs. Is there any obvious reason
why -cum is the only instance of the archaic system? May
*kom (cum) and *ko (cis) and *kwe (-que) be related
etymologically around the idea of "here-nearby-with"?