This is dead in the water. According the article, "[t]he group proposes that the shared UK functions would include the monarchy as head of state, foreign affairs, defence, national security, immigration, international treaties..." The reason that Scotland apparently wants out is they want to remain in the EU. To do so, Scotland would need to be able to have its own international treaties. It can't share the power to make international treaties with this new proposed union and remain in the EU.

this could have been a go back when they first started devolution and set up the Scottish, welsh and Irish* parliaments but the boat has sailed now.

*the n Irish already had one but it was .. closed.. when the Troubles got heated and they only were allowed it back after things had settled down again