I agree that the engineers make this far more complicated than it needs to be.
But to answer one of your original questions, the reason two 1.25's = 1.56 is because they are multiplied, not added. 1.25x1.25=1.56
If you up-size the wire by 1.25, then up-size the fuse by an *additional* 1.25 over that, you are up-sizing the fuse by 1.56 over the original Voc and Isc.
IMHO, just make sure you use large enough wire to get the minimum voltage drop you are comfortable with, then size the fuse sufficient to protect the wire size.
