I'm really liking these ideas - but of them, which one could work in a POD circa 999-1000?
When the Latin alphabet was adopted by the Scandinavians, Þ/þ was retained from the old runic alphabet. Same thing with the Goths when they (presumably) adopted the Greek alphabet.
Now, it would be very possible to just use the Early Cyrillic alphabet from the get-go. There is, however, a problem - at least in Old East Norse (not to be confused with Old West Norse, which is largely the basis of the standardized "Old Norse"), there were dental fricative - voiceless Þ/þ and voiced Ð/ð, if we use the Icelandic alphabet for a bit. Now, either Þ/þ or Fita could be used to represent both fricatives (indeed, Old Icelandic manuscripts used Thorn for both sounds, so there is a precedent) or we could split up the load between Thorn and Fita. Not with Cyrillic D - it is basically that, a stop like the English "D". Therefore, one of them is used for the voiced dental fricative and the other for the voiceless. I'll leave it to you to figure out which letter goes with which sound.
Why? (aside from the coolness factor). Cyrillic HAS a letter for the 'th' sound, namely 'fita' (basically a Greek theta). In Russian, it is synonymous with 'ef' (the phi-based character), but they would know what the Greek pronunciation was and use it for Swedish. IMO
When the Latin alphabet was adopted by the Scandinavians, Þ/þ was retained from the old runic alphabet. Same thing with the Goths when they (presumably) adopted the Greek alphabet.
Now, it would be very possible to just use the Early Cyrillic alphabet from the get-go. There is, however, a problem - at least in Old East Norse (not to be confused with Old West Norse, which is largely the basis of the standardized "Old Norse"), there were dental fricative - voiceless Þ/þ and voiced Ð/ð, if we use the Icelandic alphabet for a bit. Now, either Þ/þ or Fita could be used to represent both fricatives (indeed, Old Icelandic manuscripts used Thorn for both sounds, so there is a precedent) or we could split up the load between Thorn and Fita. Not with Cyrillic D - it is basically that, a stop like the English "D". Therefore, one of them is used for the voiced dental fricative and the other for the voiceless. I'll leave it to you to figure out which letter goes with which sound.