There are a whole host of microstates and more or less autonomous overseas territories that use the euro. If Mayotte can use the euro even though the island is on the brink of collapse, I don’t see why that would be a problem for Iceland.

I don’t think this is a problem for the EU – quite the opposite, in fact. It strengthens the euro’s position in international trade, increases the number of users and boosts the volume of trade. More customers for the EU’s financial services sector.

There are a number of countries that unofficially use the US dollar as their currency for major transactions without asking anyone’s permission (Cambodia come to mind).