Atti, I anticipate that the deals you negotiate are rather less complicated and unpredictable.
Anyone who suggests that they really know what will be the result of either a negotiation which hasn't started yet, a deal offered after approval by all (nearly) the Parliaments of Europe, or a no deal result is just dreaming. At least half of those involved are stating intentions 'if' but are stating them not as a true intent, but as a negotiating initial position. Miss S has just added a further huge pile of complications.
Start by looking at th epredictions made of the result of a Brexit yes vote by those in charge of our economy before the vote. Now put the same people in charge of predicting the outcomes.
As I have said, we need to have in place a readiness to deal with a nill deal result. That seems to me to be really quite likely as much because of the unlikelihood of the EU states reaching an agreement, as of our rejecting whatever may be offered.
To say that we cannot safely predict the outcome of any of the choices before us is bl**ding obvious. That does not mean that real attempts should not be made, but we have to be very realistic about the resources available, and the extreme unreliability of any such investigations.
When the UK voted to leave the EU, we voted for an article of faith leap into the dark. Like it or not, that was what happened.