Might the form chosen reflect personal preference or simple whimsy? I've heard that cited as an explanation for the choice of eth or thorn for the letter ðæt.
When you hypothesise that the different forms may denote oral variations, are you thinking of stressed / unstressed syllables? Or just whether, for example in ymb, the sound is an i or u (or e, which I've also seen used in place of y in ymbe)?
I haven't studied manuscripts in as much detail as you, but I just had a quick look at what I assume is the genuine first page of Beowulf, and the ymb on the 9th line down has a different form (Type 2 with a dot?) from all the other y's that I can see (Type 1?). So I would suggest the answer to your last question is a tentative yes?