But the Shetlander are Scots, you said so yourself. You can't have two indivisible sovereign entities occupying the same space, or they wouldn't be indivisible. So either Shetlanders have no rights independent of Scotland, or they were never part of Scotland in the first place.
The compromise would be to acknowledge that populations are not indivisible, that sovereignty is always a messily egotiated compromise, and to hold a series of democratic consultation by which the issue was referred to the populations in question, to hold "referenda", if you will, and naturally that means starting with a pan-Scottish referendum, because the Shetlanders are hardly able to to secede from a country that doesn't yet exist.
And as far fetched as that sounds, it's probably the approach that most Scots and most Shetlanders would favour. Surely it's better that the people speak for themselves, even if it is only to affirm their existing political union, than they be shouted down for fear of their getting carried away and doing something improper?