" What flaws are there in the various models or projection techniques used, and so on"
The models got the aftemath of the Referendum result wrong. The people using the models got earlier predictions about the ERM and the Euro wrong. (Whose finongers? Whose ears?) Also the number of parameters necessary to predict the behaviour of economic or meteorological systems make modelling difficult.
"how can anyone take seriously the claim that 40 years of legal, political and economic integration be so easily removed in a matter of a few weeks without causing serious structural damage to both parties, and with nothing meaningful to replace the previous arrangements? "
Nobody's claiming such. Damage and disruption yes. (Don't understand "structural" in this context.) But, I guess, your education,as led you to assume that the response to the inevitable problems of a no deal Brexit (e.g. all those shortages) will be passive acquiescence and roll over and die. That may, of course, be true of many millennials and some of your peers, Jim. On the other hand I think there may be many smart opportunists out there who will see an opportunity. The chances of your losing out on your Mars bar, or of diabetics not getting their insulin are remote. That's because peopele with no altruistic motives at all will make a killing (oops!) by providing what the EU has decided not to provide.
"To claim that No Deal is effectively harmless is almost a contradiction of the purpose of, and rationale behind, Brexit anyway.".
Again, no such claim (boring). The purpose of Brexit is to regain national sovereignty not to pick fights with the French or Germans. National sovereignty is not antithetical to friendly relations for trade and all other purposes with our neighbours.