If Scotland leaves the UK over Brexit, maintaining continuity in the EU will clearly be the goal, which means the timeline must be shorter than the two year timeline for Britain to leave the EU (the same would be true with NI, but that's less likely to happen; pro-EU Republicans want a union with Ireland that neither Ireland nor unionists -- pro-EU or otherwise -- want, and unionists still have a majority and the pro-EU ones are still probably more pro-UK than anything else. Brexit may increase factional tensions in NI, but it's unlikely to directly or in the near term lead to a consensus on independence.)

None of the OP points was about "when", but just some theoretic "if".

And Scotland and NI don't need to maintain "a strong independent economy"... they will be part of EU and the common market, that's the all point.