If you were born in the UK and have a UK passport you are entitled to reside in the UK. Coming back to live and purchasing a home in Scotland establishes that you intend to have your domicile in Scotland. Happy to discuss further. I hope that helps. Please leave a positive response so that I am creditd for my time.
I am not able to answer the political question as to whether Scotland will enter the EU as an independent country. All I can do is say that if you live in Scotland you will establish a domicile here. If Scotland enters the EU as a citizen of Scotland you would be entitled to take advantage of its EU membership for your business.
There is no set period. Domicile is gauged by intention, quality and circumstances. Buying a house, living in it as your main home, working from Scotland, having a Scottish will all point to you being domiciled in Scotland.
There is no black and white answer. If you are living in Scotland and your home is your only or main residence then you are considered resident. You could spend a lot of time out of Scotland on business for example but if your home address is your house is Scotland then you are resident there. The tax laws are of course specific in the number of days you have to spend out of the country to avoid UK tax but that is nothing to do with legal domicile. The list you have posted above are all adminicles of evidence that could be used to say that you are resident but there is no one thing in particular that says you are resident as opposed to not being resident. It is a question of fact and circumstance. Generally if you have a life in Scotland and call it home you will be treated as resident. If you make a will you can also declare in the will that you are domiciled in Scotland.
Then by coming to live in Scotland you will be able to do so. You at a Uk resident and presumably your wife will apply for a spouse visa and eventually indefinite leave to remain in the UK.