Your last few paragraphs are exactly why I feel Macron will say "no" this time. Logistically I think there are solutions that can help Eire in the event of a crash out. Not that I feel the UK, Eire and the EU as a whole won't suffer - but they collectively as a club can pull together to regroup.
Indeed.
Before retiring, I was a trader of actual goods (not just services), IOW buying stuff from all over the world and reselling it to the whole wide world. And like any other in that field was not only aware of the myriad of work-arounds (where tariffs were concerned) but of course used them in the instrumentalization of the passage of goods.
Swaps were a daily routine but can of course be done only with a limited number of commodities.
The issue here will be losing the enormous simplifications that an open border (and thus free trade zone) has supplied so far, irrespective of what tariffs the EU and the UK impose upon each other or decide (in some future deal) to scrap altogether. The latter being, incidentally, a solution I vastly favour for both.
Naw, the issue of control is of course made necessary by potential passage of goods from third countries into the EU via an equally potential Ulster back door. Nothing that can't be addressed by the simple instrument of certificates of origin (as in the past), but cumbersome all on its own already, not to mention that this won't make border checks completely obsolete.
Logistically - that border will be difficult to police for smuggling but that could be contained by the EU monitoring all goods and people coming from Eire - without putting Eire out of the single market. Equally, Eire could join the Schengen Agreement and who is to say some of that smuggling won't just be people but some goods coming into the UK.
What people simply don't understand, often by adamantly refusing to, is that the issue here is (far more than third country products entering the EU via Ulster and thus Eire) the passage of trade between precisely N.Ireland and Eire themselves, respectively. Including,of course, businesses that have established themselves on both sides of the (invisible) Irish border for the simplification of conducting affairs.
As I pointed out, Eire is handling Schengen already as though it were part of it anyway (there's little red tape on both goods and people) so whether it finally joins officially or not is really of secondary importance in this whole mess.
As to putting Eire out of the single market, whichever way this whole ting goes, that won't happen. It's unthinkable both on the side of Eire and that of the EU.
Finally, for the chronically deaf here, the backstop regulation was created at the instigation of the UK under May. It was agreed by both parties on the condition of not being a permanent solution and to be replaced by something that works better. That solely the EU is required to cough up whatever that replacement needs to look like is a by now a popular myth, emanating from the same bunch of habitual liars that appear to be incapable of emanating anything but.
#FBPE #RevokeA50 #StopTheCoup on Twitter: "#Brexit is like being forced to walk into the sea, while #NotMyPM tells us 'Stop scaremongering, you won't drown - you're a fish' and #Brexiters can't decide what kind of fish they want to be. But they definitely DID vote to be a fish, they know that much 
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