Chaddz3 is more of the expert on this, but I'll share what I can ...
For lack of a better term, Ford "discourages" this type of modification. I would think you could install a different BCM and get the dealer to enable fogs in IDS - but only some dealers know how to do this, and only some of those are willing to do so.
In addition - on the Focus, the S didn't have fogs, but the 2012 SES and 2013 Titanium did, so there are BCM models with them. I'm assuming there is a C-Max Hybrid with fogs, so there should be a BCM that would work, but maybe not.
Bypassing the BCM is almost certainly easier.
Electrical changes are somewhat of a concern, but likely over-rated. (I have heard of people running 500W RMS or more subwoofers in a Prius, for example - not the same car, but same principle.) If the uplevel C-max hybrid has fogs, I wouldn't be concerned about adding them to the base model.
On the Focus Headlight switch:
Chaddz3 has stated that the switch sends data (not power) to the BCM. So you can't use the switch as is.
The easiest solution is what Chaddz3 did where you install a toggle switch somewhere else.
Outside of that - it would be simplest if Ford had a switch without a button on the left, but the non-Fog switch has the dash dimmer there. I would imagine there would be someway to mount some type of contact switch in place of the fog switch and/or to design a circuit that read a press of the existing switch and closed a standard switch - but I don't know how easy that would be.