E-Ching vreit:

>We know some "c"s became "ch" because we say the words that way now,

and as soon as the Normans invaded England the Norman scribes spelled
the "ch" words differently from the "c" words,

Just a thought (this is your expertise, not mine):

Might the Normans have pronounced some c's as s's, e.g. in "centre",
and wanted to separate those from the English "ch" [tS] sound? The
Normans may have had [tS] themselves, spelled "ch", as in "champion";
but they'd have wanted to continue spelling that "ch" (and thus
respelled the native English words accordingly), and retain the "c"
for etymological purposes, as it is today.

> Old English teachers will make you learn to pronounce the two "c"s

differently so that you know there can be a difference; luckily Old
Norse doesn't seem to have this kind of exception built into the
system.

I beg to differ :) The difference between the "k" in "kalt", [k],
and "kært" or "kelda", [c], may not seem great to you, but it's still
there. It comes automatically to most Germanic-speakers, but not to,
say, Spanish-speakers; Spanish has "calor", [kAlor], and "queso",
[keso], not [ceso].

Óskar