I'm not sure that Scotland and Northern Ireland would automatically be allowed to "stay" in the EU - wouldn't they need to reapply for membership as new entities?
As far as I understand it they would have to reapply, but if I recall correctly back when the referendum on Scottish independence was a thing the EU said that they'd fast track their application, whatever that means.
Again, if I recall correctly.
Scottish independence will happen eventually.
A neat solution would be for England and Wales to jointly declare independence, leaving a UK consisting of Scotland and Northern Ireland.
Yes but it would be a mere formality as they are already part of the EU. Especially for NI which would be joining an existing EU state, not getting independence.