Check out your accordion dimensions and weight in its case. Some airlines will allow them on as hand baggage if they fit the above specifications. I have flown my Saltarelle button accordion to France and Spain from the Uk several times as hand baggage, and can get travel docs etc into the case, though I guess it’s smaller than yours. Very bad for the reeds to go as hold baggage, as far too cold!
How are you getting to Orkney/Shetland from mainland Scotland? If by ferry, no problem, but if by air, check out Loganair for their rules.
Finally, the other option to consider is leaving your own precious accordion at home and looking to borrow or hire one over here. Scotland is FULL of piano accordions everywhere! (Button boxes much rarer!)