it isn't just about paperwork, it is about logistics, we and the EU currently do not have the infrastructure in place for changes of this magnitude. We may care about that but I'm guessing the EU will just have a collective gallic shrug and not give a toss, and why should they ?
It should be obvious to anyone with half a brain
that the EU is not going to spend a wooden cent on anything to do with Brexit until it's been inked signed, sealed, and possibly passed by a vote in the UK HoC.
Probably because - unlike the UK government - the EU doesn't want to waste taxpayers money.
I suspect they already got the next stages planned - and if I am right, it will get even more ironic (if possible).
Let's assume (and that gets bigger daily) we do manage to ink a deal on Brexit by October 2018.
And amazingly it is approved by the various levels of EU governance by March 2019.
It could be at that point, the EU starts to build the UK-EU infrastructure it requires. No doubt it will have plenty of time to do so, since there isn't a cat in hells chance the UK will have.
This could string "Brexit" out for years. Obviously the UK would be the main casualty, rather than the EU, but we chose to cut our nose off.
Of course all the whining from Brexiteers will just get the response "well what did you expect ? You told us how inefficient the EU was ..."