The will would have to be examined by an English solicitor to confirm whether it is valid under the law of England. If the deceased wasn't domiciled in Scotland then you can't apply for Confirmation in Scotland irrespective of where the will was made.
That is correct. There would be no need for verification in England if the deceased died domiciled in Scotland.
No, it's not time based. Time is a factor but if the person considers that he is domiciled in Scotland and that one day would return there then that would be sufficient.