Lump Jockey: It would be great to equip all the aircraft with CatIIIc equipment, however different aircraft of different generations ie 737-100 and 737-800 have different though similar systems, even within the same generation depending what was specified by the original purchaser. Think of it like buying a car , you have all the options for all the toys from new, depending on you budget, but if you buy secondhand you take what you get. Its not just a matter of slapping in a 'CatIIIc box'. To make the aircraft C3c would involve changing many many components and avionics systems, and in the end would be hugely expensive for the one or two occasions in the year when it is needed.
Yes if one of the pilots isnt qualified the aircraft will divert. Flying as a whole is covered by a multitude of backup systems. What would happen if something went wrong very late and low on a C3 approach , ie the qualified pilot collapsed? Also the approach isnt just flown by one of the pilots it is flown by both, generaly the First Officer (F/O)will fly the aircraft on instruments to the decision height ie 50ft (or lower) whilst the Captain monitors the approach and looks out the window. At decision the F/O calls decide and the Captain will say land, if he can see the runway and takes control whilst the F/O remains on instruments, or he will say Go around and the F/O will fly the go around. Its not a one man band!
Hope that explains things a little better....