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.