Your weather assessment is correct.
There will be a decent cold front late this week but
the surface high behind it, dropping from Argentina to far
se Brazil(along the coast), is coming in much too far east for it to bring in enough
cold, dry air from much higher latitudes.

If that high was instead moving from nw Argentina to Paraguay/Bolivia, then
to MG/coffee land south winds ahead of it could drive in
an air mass capable of night time frosts.