Found something to support my Tiber-in-the-north-of-Ceres Theory.
In CuF, Cyril says the following: "If they take the Elinarch, they'll run right through Placidas heartlands on the north side, and with the river protecting their flank, they'll be able to lay waste to Ceres' lands on the south."
This indicates to me that the Tiber hast to be at least in the northern part of ceresian lands, if not form the border between Ceres and Placida.
As i see it, the Canim wanted to control the Elinarch because the Alerans otherwise could deploy forces in their back, not because the want to get to Alera Imperia.
That's indicated by Ehren saying "... It's obvious that they must control the Elinarch, or risk being attacked from several directions."
iirc the way to the capital being free for Kalares Legions is connected to taking Ceres. The Elinarch isn't mentioned in that case. So it could well be north of Ceres.
I totaly agree that atticas lands have to reach close in to Kalare, but that does not necessarily mean that the City of Attica itself needs to be so close.
The Blackhills... well it's a mystery :-)