I would recommend consulting Prokosh Comparative Germanic Grammar.

It’s a very old reference and may not be online, but what my germanic linguistics professor used with us Germanic linguistics grad students. He was a world renowned expert on the various futhark versions.

https://en.wikipedia.org/wiki/Elmer_H._Antonsen

Prokosch's book is still available

https://a.co/d/0u4S28Y

AFAIK thorn and eth were used in free variation, as there is rarely any possibility of confusion.

I'm not huge on taking the Wikipedia entry at face value and prefer to look at the references used for the entry. In this case the reference CHAPTER 25 TYPOGRAPHY AND THE PRINTED ENGLISH TEXT, page 6, does mention that y/ye was used in place of both eth and thorn.

Mispelling or incomplete font set for printing press possibly?

Note modern English collapsed the distinction into simpler orthography, “th” for both eth and thorn, so simplicity in spelling certainly happens.