Fascinating! It neatly lines up with /z/ possibly being [ʐ], the ideal candidate for the later change to /r/.

I think for the North Germanic rhotacism it is not that easy. There are several more phonetic AND phonological circumstances that need to be factored in and explained. I recently gave a presentation where I listed them and came up with a hypothesis: www.researchgate.net/publication/...

(PDF) The phonological properties of the rhotacising z/ʀ-phoneme in Ancient Nordic

PDF | The rhotacising phoneme *z/ʀ started in Proto-Germanic as a voiced strident fricative */z/. Where not assimilated the descendant (by convention... | Find, read and cite all the research you need...