Actually, I have another question regarding skjöldr:
*skelduz > *skealduz > ON skjöldr "shield"
I'm pretty sure the process that turned the root vowel "e" into "ea"
is an example of breaking. I think the "e" turned into the sound "i"
which transformed into the semi-vowel "j" to produce "skjöldr" (right
?). However the dat.sg, nom. and acc. pl have the root vowel "i"
instead. How did this happen ? My guess is that the root was subject
to i-mutation, as in *arni > erni, but the vowel cluster "ji"
simplified into "i". Do you confirm ?