The union of the UK and the union of the European Union are not the same thing at all.
Scotland isn't being 'allowed' to leave the UK for a start. Not really a union then.
The amount of planning and pre-work done (much more by Scotgov than by those involved with Brexit) will make a difference
Scotland can follow a well defined map from others who have won their independence from the British Empire
A Scotland/England border would maybe solve some Irish border problems and be easier to manage in practice
Currency would be whatever Scotland chooses including Euro which I wouldn't have a problem with especially given the state of Sterling (done on purpose by those wishing to profit / disaster capitalists)
Scotland wants immigration and would welcome incomers from anywhere including Wales and England
If Scotland went back into the EU then many businesses currently in Wales and England may choose to base here
Scotland's people working for independence would not be looking to complicate things like the Westminster politicians are (looking for no deal Brexit), running down the clock towards a completely arbitrary date they chose themselves, etc)
Scotland has energy, food, oil, exports that other countries want