sh_ and ch_ compose the same words

[Davidsch]

I do Not understand these conclusions. (In my bowl apples and pears are both fruit.  chedy= shedy)

Let me ask you Torsten, in your view, could you answer :

from the medieval manuscript viewpoint, for a random hypothetical invented alphabet

If we would use the letter c similar to the letter k, but have a slight preference for one of them,
would you then more likely see if they are 

a) different
b) they behave similar

c) neither

(of course you could try take other letters such as s&z, u &v, v&w b&p etc.)