I think most of what you said is certainly possible. I don't know how you square 1 with 2 tho, either the EU was important or it wasn't--> there is either a bunch of economic fallout and UK reclaims its "sovereignty" or there is very little disruption substantively to the trade agreements.
i mean, it really just depends entirely on what agreements replace it. i kinda agree with fsd that the new structures will pretty much keep almost all of the trade continuity and probably also most of the movement of labor/exchange of social services.
My biggest fear is that European separatists start to fracture all the nation states at the same time that the international institutions fail and that it starts with the UK (Scotland, Northern Ireland) then spreads to Spain (Catalonia, Gibraltar, basque) then to Italy (North/South) etc.
i think your fear confuses cause and effect. i would also conjecture that scotland would only leave the uk if they are guaranteed entry into the eu, so that would actually strengthen the eu. i think your other proposed fractures would also strengthen the eu (small states need the common market more than large states. really, the idea of splitting off these little micro nationalities into states is only viable within a framework like the eu).
best case scenario, the uk moves to eu lite by another name, the continental members remain and the eu becomes more responsive to member states complaints. it's not hard to see this being a net positive long-term.
i think it's a much more scary situation than that. right-wing leaders in Finland, Austria, the Netherlands, France and Hungary are already calling on in/out referendums in those countries. the EU was set up to stop Europe from tearing itself apart after WWII by linking them economically, considering that the power vacuums that existed after WWI lead directly to the next great war. if the stronger, more populous and more prosperous countries leave the EU, it has two really negative consequences in my mind. The first is that it exacerbates the Syrian refugee crisis by leaving countries that are wholly ill-equipped to handle it all alone. The second is that it will more than likely embolden Russia to become more aggressive towards EU countries in general and the Baltic states specifically, and there is a high likelihood that Russia will do to those countries what it's been doing to Ukraine for the past 3 years.